Phinney on Fonts About Thomas & the blog Phinney on Fonts main page

Picture of ThomasThomas “my other car is a sans serif” Phinney on fonts, typography & text. Geeky troubleshooting and info for font developers and users. Consulting & expert witness for fonts & typography.Read more...


« Point Size and the Em Square: Not What People Think

It’s easy enough to determine that a point is 1/​72 of an inch, and used to be about 1/72.27 in the days before digital type. But the challenging question is, when you look at printed type on a page, what part of a 12-​​point font is 12 points high? The short answer is “none.” Seriously. For metal type it’s the “body” which is not something you see in print, and for digital type it’s the “em,” which is completely virtual.

[I have been working on this piece on and off for months, and I keep on thinking it needs more graphics. But in the interests of getting it out there, I’m letting it go as is, because I think most of this is clear enough even without. If anybody wants to point me to a link or create a graphic to illustrate a point made here, feel free!]

Font Size Measurement Confusion

The background to this is long and complicated, so I hope you’ll forgive me if I first explain how this is the question that just refuses to die, and the confusion it can cause… in painful detail.

Historical Background

Back in the days of metal type, the answer was simple, even if it didn’t relate to anything one saw in the printed output. The point size of the type was simply the height of the metal body the type was cast on. Additional line spacing was added by means of thin strips of lead between the lines, hence the term “leading” (pronounced “ledding”) for line spacing.

Metal type, showing point size

Above is shown a piece of traditional metal type (photo courtesy Daniel Ullrich, licensed under Creative Commons Attribution Share-​​Alike 3.0). The added red bracket shows the body height, which one would measure to determine the type size.

In metal type, without leading, the distance from the baseline of one line to the next would be the same as the point size. However as you can see in the example, once the metal type was printed, there was no direct means of knowing what the original point size had been, unless one also knows either the original typeface or the amount of leading used with some certainty.

Today’s Answer & Implications

In digital type, the font’s letters are drawn on a grid, where an arbitrary number of units (often 1000 or 2048) are set to equal the “em” which is then scaled to the current point size for output. So to get 12 point type in print, with a 2048-​​unit em, that digital space is scaled so that the 2048 units in the design space are equal to 12 points. As Karsten Luëcke put it in a recent discussion on Typophile:

In digital type, the EM does not refer to a “real” box. You better consider the EM as a yardstick – an abstract letter-​​height yardstick which establishes a link between micro and macro level, between font-​​internal unit system and font-​​external unit system: The font-​​internal unit system is defined via UPM, i.e. as the number of units per EM. It is the letter-​​design grid or resolution. The font-​​external unit system may be typographic point, millimeter, pixel, etc. And this abstract EM serves to project the font-​​internal unit system onto the font-​​external unit system.

An example. You have a font with 2048 units per EM, internally, which is to be projected on 12 pt type size, externally. So 12 pts = 2048 M-​​units or 1 M-​​unit = 12/​2048 pt.

So to image the font at 12 point, one scales the abstract EM to equal 12 points.

The catch for purposes of measurement and standardization is that while there are some restrictions on how large one can draw letters in the design space, there is no necessary and required relationship between the size of the letters and the em. On average, with Latin-​​based languages such as English, the “cap height” of capital letters is about 70% of the point size, and the “x-​​height” of lower-​​case letters is about 70% of the cap height, or about half the point size. But (and I cannot stress this enough), those are only averages, and there is no technical requirement whatsoever that one be close to those averages. Indeed, x-​​height relative to cap height is one of the ways typographers describe typefaces (“high x-​​height” vs “low x-​​height”).

[UPDATE : I did some research for a client, and verified that as expected, cap size varies substantially between different fonts. In my sample, cap size was most usually 62%-78% of the em square, averaging right around 67-​​70%. Or to put it another way, if you take an “average” font printed at a given point size, other fonts at the same point size will commonly have capitals as much as 10% smaller or 10% larger than the capitals from the average font. At the extreme you can find fonts “in the wild” with caps barely over half the average size! (I expect you could also find fonts with caps close to half again the average size, but I wasn’t looking so hard in that direction.)]

Moreover, the Zapfino example given earlier shows how a given font could be at a radically different size relative to the point size and still be a legitimate font. Indeed, anyone knowledgeable in modifying fonts could in a matter of minutes, take almost any font and create a modified version, with the only visible difference being that text at a given point size is only a fraction of the size.

What About the Web?

The web can use points, but just defines them in terms of pixels. It has inherited the Windows definition of that ratio, so on the web by default 1 pt = 4/​3 pixels, so 12 pt = 16 pixels (but see below).

It used to be that Mac browsers used the Mac relationship of points to pixels, which was one-​​to-​​one, but that has been abandoned just a few years ago. so at least points vs screen pixels are now consistent across platforms, though how big a point is on screen (or a nominal browser pixel for that matter) depends on your screen resolution, what zoom level your browser happens to be set to at the moment, and (on Windows) whether you have set something other than the default screen resolution of 96dpi.

But the relationship between pixels and points is broken in some browsers on Windows (such as Internet Explorer 7 and earlier) when the user has a non-​​standard resolution set. For example, if you actively tell Windows your screen resolution is 120 dpi instead of 96 dpi, that means that point sizes get multiplied by 5/​4, but sizes in pixels do not. So at 120 dpi, a font set to 9 pt will instead show up at 15 px, but a font set to 12 px will still be 12 px, and now smaller. Arguably this is a reason never to do font sizes in px. (Bitmapped grapics generally are not scaled by the 5/​4 ratio in browsers, but they are in other apps such as Word or the usual graphics previewing programs.)

This may get even less standard in the future, as CSS 3 is threatening to make pixels a truly imaginary thing, always equal to 4/​3 the point size. This would cause pixels to scale into virtual pixels when non-​​standard resolutions are set.

Of course, some users (like me) are constantly changing the zoom level in their browsers, which also plays hob with any notion of fixed sizes for points, though at least relative sizes are maintained by browser zoom.

Things get kinda weird on the web, in another regard. CSS can use “ems” as a measurement unit. Okay, that makes sense, right? I mean, why not set an indent or margin in ems? No problem. Where it gets weird is that you can set the type size in ems. Now, logically based on the “normal” definition of the em, this makes no sense, because the size of an em is always the same as the type size, so the size of the type is always one em. But CSS allows you to break that assumption by setting an em to some specific number of points or pixels, and then setting the type size to some multiple of that. It gets even weirder, actually, because you don’t need to define the em in the first place. If you don’t define it, the standard browser assumption is that one em = 16 pixels (Firefox and possibly Chrome), or 12 points (Internet Explorer). The difference between IE and the rest doesn’t matter with default Windows resolutions, but it gets interesting at non-​​standard Windows resolutions because IE then scales the default em, while Firefox does not…. Ouch.

[Note: edited and expanded this section several times on 21 March 2011 to better reflect system scaling setting issues. Thanks to Beat Stamm for pointing out the omission and helping me with details I hadn’t yet encountered.]

How to Legislate Type Size Today?

First, a disclaimer: One can implement reasonable precautions, but it’s not possible to stop determined people with sufficient knowledge of fonts and typography from creating customized fonts, which can in turn be used to create either illegible documents, or disclaimers that most people would never read. To even attempt to cover all possibiities would probably yield many pages of added law, which frankly somebody like me could probably still find a loophole in with a moderate investment of time and thought. What reasonably can be done, however, is to make the laws tight enough that it would take significantly more expertise, creativity and effort to work around them than is currently the case.

So what variables does the law need to control when it wants to legislate a minimum size and legibility?

Most common system fonts a reasonable person would think of using would mee these requirements, including Times/​Times New Roman, Arial, Helvetica, Courier/​Courier New, Verdana, Trebuchet, Georgia, Calibri, Consolas, Constantia, and Corbel.

Of course, I’ve only addressed the font size part of the equation. There are many other components to legibility of text in print, such as line spacing, letter and word spacing, line length, and the color of the text and the paper.

[EDITED various times to clarify minor points and improve wording.]

ADDENDUM 16 August 2012:

This stuff just doesn’t go away! A recent decision of the Michigan Supreme Court hinged on exactly this issue. The underlying subject matter was the hottest state political issue of recent years, an attempt to put in place a ballot measure that would in effect stop the ongoing removals of collective bargaining rights for folks doing business with cities. Here’s the Detroit Free Press about the case, and the actual court decision (including concurring and dissenting opinions).

21 commentsto “Point Size and the Em Square: Not What People Think”

  • March 18, 2011
    Lindsay wrote

    I love that you’re writing about this, I know a lot of non-​​typographers (some of them designers) simply cannot wrap their minds around the idea that point size does not necessarily indicate the size of printed text.

    Further, I commend you for offering the legal system some well-​​thought-​​out options… Although I do find it strange that they haven’t just instigated mandatory minimum heights. I have designed wine labels, and I know that the legislative bodies that check alcohol labels do have minimum size requirements set in mm (2mm, to be exact) for certain items of information. Of course, they don’t set any width requirements, so if you look at a wine label you’ll often notice a really awful vertically stretched warning label in all caps (with negative tracking) somewhere on the back.

    Personally, I think the 2mm min actually does a disservice, because the people who enforce the rule are not designers or typographers and aren’t interested in legibility/​readability, only code enforcement. I’ve long thought a warning label in u/​l, even with x-​​heights less than 2mm, would serve the code’s purposes better.

  • March 19, 2011
    Mike Perry wrote

    Fascinating article! I’m starting to understand why some fonts are so much bigger than others with the same official font size.

    For laws, a useful and simpler rule might be to require that exactly the same font and font size be used for all text portions of the document. That’d mean that to render illegible the parts schemers don’t want read, they’d have to make the parts they want read also illegible.

    That could be coupled with a minimum font size rule and perhaps one requiring that a large-​​print version to be available on request for those with vision problems.

  • April 3, 2011
    Francisc wrote

    with Latin-​​​​based languages such as English”

    English is not a Latin-​​based language. Only its alphabet uses Latin characters.

    [Sure, I should have said “Latin-​​based writing systems” instead of “Latin-​​based languages.” I shall revise that for clarity.—T]

  • April 3, 2011
    Craig Schleunes wrote

    To Mike –

    The SEC has a rule, 482(b)(5), that does what you are suggesting. Basically, it requires advertising disclosures to be “…in a type size at least as large as and of a style different from, but at least as prominent as, that used in the major portion of the advertisement.”

    [I like that. Probably better than trying to get into the kind of detail I was doing. It is simple and to the point. There is some grey area and room for interpretation, but anybody doing anything really egregious will be clearly in violation of the rule. Though I might want to cover “legible” as well as “prominent”….—T]

  • April 4, 2011
    Francisc wrote

    I really enjoyed the article Thomas, I should have mentioned that in the previous comment.

  • April 5, 2011
    John wrote

    Browser differences and customizability do make the standard point size anything but. However, at least in print, a typeface’s point size is measured from the top of the tallest ascender to the lowest descender.

    [Sorry, but no. I mean, you can measure that distance, but it is not necessarily the same as the point size of the type used to print the page, neither in metal type nor in later technologies. In metal type that distance would be equal to or less than the point size, and usually pretty close to the point size. But in digital type, the relationship has been broken. Else what I described with Apple’s Zapfino would not be possible, would it? Digital fonts are not only for screen usage….—T]

    Points “equal” in size should have these line up if placed side-​​by-​​side.

    [Not necessarily. That is the whole “point” of my post. There is no physical thing you can measure from the printed type to determine the point size.—T]

    However, then factor in the “x-​​height”, and the “illusion” of type size differences can really get pronounced. Lofty, scripty fonts will look “small” (big ascenders and descenders, mixed with small x-​​heights) and modern screen sans serif fonts (Arial, which has a large x-​​height) may look so much larger when compared, even when they are the same “point size.”

    [Absolutely. Differing x-​​heights contribute substantially to visual differences in type size, even aside from the issue with point size not referring to anything you can physically measure.—T]

  • April 8, 2011
    Michael Olivier wrote

    My brain hurts from reading this article. I will need to come back (possibly, again and again) to wrap my head around it…since I am a designer;). Great writing, though. Thanks!

    Also, noticed in your header that a search field or something gets cut off in the About section (below the copy). I can just see a little sliver of it and put my cursor in there. I am on Mac OS 6.x using Firefox 4.

  • April 22, 2011
    Sylvain Galineau wrote

    Great post ! One nitpick (OK, maybe it’s not a nitpick for me): pixel-​​sized fonts in IE no longer grow relative to pt-​​sized ones when you increase your screen DPI setting since IE8. As you know, a CSS pixel is 1/​96 of an inch and browsers have treated it that way for a number of years now. So the pt and the px have a fixed relationship that is maintained regardless of DPI; this is certainly true on Windows today, at least.

    [Thanks, I will amend that point! — T]

  • July 21, 2011
    Kristoffer Tronerud wrote

    Hi Thomas,

    Very interesting stuff, but unless I’m missing something (I had to skim because I’m trying to settle a specification argument here at work) you never provide some sort of practical condensed answer to your original question (Yes, I know you said thee wasn’t one, but…) — “what part of a 12-​​​​point font is 12 points high?”… I’d like one, thanks ;-D

    [Urm, but the answer is, NO PART of a 12-​​point digital font is 12 points high. Or you could say “the em square, but that’s invisible and imaginary.” – T]

    Kris

    Kris Tronerud
    Graphic Designer/​Technical Illustrator
    Smiths Medical/​MROC
    Tel: 781-​​763-​​9356

  • July 21, 2011
    Kristoffer Tronerud wrote

    PS-​​

    It would seem to me that a “quick” answer would be the distance from the top of the Cap to the bottom of the Descender –

    [If by “quick” you mean “approximately right most of the time” then sure. For that you could also take 1.5x the cap height. Or twice the x-​​height. Any of those should average about the right number in an average font, and be within 10-​​20% of correct in >90% of fonts. But just remember that there is no required relationship there. – T]

    Kris

    Kris Tronerud
    Graphic Designer/​Technical Illustrator
    Smiths Medical/​MROC
    Tel: 781-​​763-​​9356

  • September 23, 2011
    Marco wrote

    Thomas, thanks for this interesting article.
    Was trying to figure the following situation. Given a font and a size in point (say Arial 12), since points are a measure of height, if I modified the width (letter spacing or proportion) can we still consider an Arial 12 in legal terms?

    thanks for your opinion.

    ML

    [I can’t speak for the legal part of this. Modifying the letter spacing or horizontal proportions doesn’t affect the point size in technical terms. But I imagine somebody might question whether your modified font was still really “Arial,” perhaps? — T]

  • November 9, 2011
    Ian wrote

    It’s easy enough to determine that a point is 1/​​72 of an inch”

    Perhaps on paper, but when screens are different resolutions with different dot pitch, that is not easy enough to determine at all.

  • January 25, 2012
    Lori wrote

    The point size of a typeface is the measurement from the top of an ascender to the bottom of a descender. So you can’t use a cap “H” like in the example given to determine point size. You need two letters (e.g., hq, you would measure from the top of the h to the bottom of the descender on the q). “M” represents the width of the capital “M” in a given font. So if you’re setting some copy in Arial, and you need an em-​​dash, that dash is supposed to be the same width as that of Arial’s capital “M.” The explanation is the same for “N.”

    I used to work for a hot-​​metal an Penta system typesetting company, and the way we always verified the point size of a typeface as well as leading was by using a tool called a type gauge/​graphic art ruler. It’s a transparent guide that you lay on top of copy to measure point size and leading. I don’t know how long they have been in use, but they’ve at least been around since the early ’70s, if not longer. Do an Internet search and you can see what they look like. I’ve always used an all-​​in-​​one tool, but they are also sold separately. I don’t know why, because if you need one, you will need the other.

    [Thanks for taking the time to write. I hope that what I am writing here doesn’t come across as too harsh. The whole point of my piece is that things were never as simple as people would have liked, but it has gotten much worse with digital type.

    Of the six sentences in your first paragraph, all but the second one (“So you can’t use a cap”H” like in the example given to determine point size” which is true) are false statements. To take apart your own examples with actual data, in Arial the ascender-​​​​to-​​​​descender distance is 1873 units (for top of d to bottom of p, could be 1922 from top of f to bottom of g). That is less than the em square of 2048 units. In other words, the distance you say will equal the point size is in fact about 6% to 9% smaller (depending on which measurements you use for ascender and descender). The advance width of the M is 1706 units, but that of the em-​​​​dash is indeed the expected 2048 units. The N is 1479 units wide, while the en-​​​​dash is 1139 units (interestingly, more than the expected 1024 units, but still unrelated to the cap N). Times New Roman, which is pretty carefully based on the metal Times typeface, is even a bit smaller than Arial on the em-​​square. That is, the difference between the ascender-​​to-​​descender distance and the point size is even greater than it is with Arial.

    I have indeed used a type gauge myself. I have a couple in my home office. They show measurements in points and picas, rather like a ruler. Some of them also use a common typeface such as Helvetica to show size of type, using a letter such as the capital “E.” However, as discussed, the height of the cap “E” in one typeface will not necessarily be the exact same size as the height of that letter in another. For example, Comic Sans has a cap height almost 10% taller than Arial, while Calibri has a cap height about 12% smaller than Arial. So the type gauge’s “E” measurement is only exact and accurate if you are using the same typeface. The same thing is true of ascender and descender sizes: they vary between typefaces. In fact, there is no measurement you can make of anything in a font that is the same from one font to an unrelated other font. They just don’t work that way. Certainly not in digital fonts. Even with metal fonts you couldn’t take printed output using an unknown font, measure some distance, and declare that to be the point size of the type. It has never been true.

    What could you actually measure? In metal type you could be guaranteed that the ascender-​​to-​​descender distance did not exceed the point size. But it could be less. One other thing you can say is that with metal type, if it was “set solid” (without leading), at a constant point size, the distance from one line to the next of any given attribute (for example, the baseline) would equal the point size of the type. But that relies on knowing it was set solid. You could be sure that the point size was no more than that distance, but it could be less.

    — T]

  • January 30, 2012
    Thomas Byg wrote

    an intellectual CIRCLE-JERKLEGIBLITY is the issue…all legal papers must be in one font… at one standard size…done

    [So, which operating system or font vendor are you going to give a legal monopoly to? And even if “one font, one size” works for “legal papers” there are cases where we’re talking about advertising, or product labels, or… all sorts of other things where some degree of font freedom is seen as desirable. It just ain’t that simple. — T]

  • September 5, 2012
    Stephen Samuel wrote

    One approach might to be reasonably vague — require 12 points high, reasonably proportional, and easily readable. Then list a few well known exemplary fonts which explicitly fit the requirements. Say, Times New Roman, Century Schoolbook, Geneva ….

    That way, a scoundrel can’t claim that (s)he doesn’t know how to obey the law, the definition of ‘easily readable’ can be left to a jury, and someone who doesn’t know fonts (and doesn’t try to work around the law) is highly likely to be able to obey it.

  • September 12, 2012
    Charles Whitehead wrote

    Mmm. Very confusing.
    I think the issue of caveats should be with one standard font of a standard size. No arguments, no leeway. One font, specified in law….and, make it international.

  • September 12, 2012
    Thomas Phinney wrote

    There are a few problems with that idea. Not insurmontable, by any means, but serious considerations.

    1) What typeface are you going to give a monopoly to, and how do you choose it? I suggest we use an open source choice to avoid blessing a single commercial enterprise. Let’s say we do that.

    2) “Make it international.” How international do you mean? Do we need one font that covers all the world’s major languages? I will assume so, but making this work with the other requirements is a problem.

    3) Do we need a full family of four fonts including bold, italic, and bold italic? I would assume so.

    4) Let’s say that we want a typeface that is particularly legible. When I think of what is unacceptable, I am looking at you, Helvetica and Arial. Times isn’t all that for legibility, but it is adequate in in print. Not great on screen.

    5) Let’s also say that we are going to need both a serif typeface for print and a sans serif or slab serif for screen.

    Now, the problem is, there are no typefaces that meet all these requirements. Heck, I can pick two or three of these requirements, and I believe we will find that there are zero options: open source, all the world’s major languages, comes as a four-​​member family. Nope!

  • January 31, 2013
    Louis Gotlib wrote

    A bit of tangent here-​​ I am a slide rule collector and I recently came across an item known as a Wilson’s Cylindrical Calculator for Rule Work. I know that “back in the day” it was used in helping plan typesetting, but I can’t tell any more than that. I can send some scans to anyone interested. If anyone knows more about this item I;d love to learn about it. As far as I can tell, there has been one sold via ebay (the one I just got) and on auctioned in 2007. Here is ebay link. Thanks.
    http://www.ebay.com/itm/Rare-Wilsons-Cylindrical-Calculator-1915-/380561683920?_trksid=p2047675.l2557&ssPageName=STRK%3AMEWNX%3AIT&nma=true&si=6fLP8%252B0u%252FYxdYuCDUtj1Rsb4b2k%253D&orig_cvip=true&rt=nc

  • January 31, 2013
    Thomas Phinney wrote

    Very cool! I am not familiar with this particular device, but clearly it was used to plan how much text would fit on a line and/​or page. This is similar to “characters per line” tables that were once published. What makes them complicated (besides the fact that they have to make some assumptions about the mix of letters in your text) is that every font is different.

    I strongly suspect that the “series” setting on the left in your device is a numeric code for the typeface, from a specific manufacturer. Possibly for the Monotype machine or American Type Founders—they both used numbers in exactly that fashion, and both had numbers in the right range at the right era. For example, 93 was the Monotype series number for News Gothic Bold, released ~ 1908.

  • January 22, 2014
    Adrian wrote

    With reference to printed documents, for legal purposes, I do not understand why the law doesn’t simply use millimeters to define character dimensions (probably a minimum height for upper and lower case letters, plus a minimum width). To try to make a legal definition using points when, as the article makes clear, they have no need to relate to actual real world dimensions is futile.

  • January 22, 2014
    Thomas Phinney wrote

    You could measure any of those things in points, btw. It’s just that the notion of an overall general “point size” doesn’t map well to any specific thing.

    Any other system of measurement will map better to real world concerns, but has the significant limitation that it is no longer trivial for a person creating a document to know that they are in compliance. Overall point size is the thing you enter in an application.

Leave a comment