The magazine of the Melbourne PC User Group
Measuring Type
Major Keary


Major Keary has been delving deep into the attributes of type — as you do — and he has written a quick lesson for all the hungry minds

From time to time it is handy to know the size of a font used in some printed document or publication. There are three typographic measurement systems: standard Anglo-American, one that may be called Adobe, and Didot.

In each system type is measured in points, but what does one measure to find the size of a particular font?

How Big Is a Point?
 

The dimension of a printer's point is something of a pre-metric moveable feast. The system was invented by Pierre Fournier, a French type founder, in 1737; his unit measured 0.349 mm. Later on (about 1780) Franςois-Ambroise Didot - another French type founder - came up with a new system that used a unit of 0.3759 mm - 72 didot points to the French inch - with twelve didot points to the cicero (sometimes called douze or didot pica); the didot point system is a standard used in Europe.

The Anglo-American System

In 1886 the American Type Founders' Association adopted a standard point of 0.013837 inch (0.3515 mm), which was later (1898) recognised in the United Kingdom; 0.013837 inches is the equivalent of 72.27 points per inch, but Adobe decided that there should be exactly seventy-two points to the inch. The result is a dual system: 'standard' points (72.27 per inch) and 'PostScript' points (72 per inch, or 0.3528 mm). Twelve points (of either kind) make a pica.



Figure 1. A diagram of metal type.

The difference may be very small, but is visible when comparing horizontal measure of 180 points (15 picas, or about 65 mm), and is quite significant over the length of an A4 page. The issue is discussed by James Felici in The Complete Manual of Typography. Who cares? you might ask. Anyone involved in precision typesetting has to be sure of which system is in use. For example, inserting text or other data on a pre-printed form where the new material has to be exactly aligned with existing elements. Typesetting scientific material often calls for precise placement of typographic elements.

Donald Knuth's TeX typesetting program provides for both standard points (72.27 per inch) and what he designates as big points (72 per inch). TeX also has the capability of setting type in increments of 65,536 small points to the standard point.

Anyone interested in the terminology, and dimensions of various typographical units, should visit http://www.oberonplace.com/dtp/fonts/point.htm and follow its links.

What Does One Measure?

Some books on typography say that the size of a font is measured from the ascender line to the descender line. That means from the top of letters like b to the bottom of letters like q.

That is not accurate. If you print out the letters qb from a 72-point font and measure the distance between the bottom of q and the top of b (See Figure 2), you will find it is 66-points, or very near to that. So, what has happened to the missing six points?
 



Figure 2. Sample of 72 point Type (which may or may not be precise, given the number of
untested stages this will go through before you can measure it).

The point system was devised for metal type. The metal blocks, or slugs, on which characters are cast generally leave space (called the shoulder) so that the letters don't bump into each other when set. The size of a font is the measure of the slug, not the letter that has been formed on it. The amount of space varies between type foundries, and even between typefaces.
 

The same requirement to prevent collisions dictates that digital type has a bounding box that leaves space around each character. It is the height of that bounding box that defines the point size of a digital font. Unlike metal type, the bounding box horizontal dimension is the same for all characters in a given typeface with a separate 'box' that determines each character's width.

To make things more difficult, the size of the bounding box cannot be gauged for different point sizes. The only way it can be seen is by opening the font in a font editor, but even that doesn't help because the bounding box dimensions change for each point size.


Figure 3. This is an upper case
'M' displayed in Noah; the
bounding box is shown with the (narrower) character-width box.


Figure 4. This is lower case 'l' displayed in Noah. Note the
bounding box is the same as for upper case 'M', but the character-width box is proportionally narrower.

For normal body text - between eight and twelve points - measuring the ascender/descender distance is a reliable guide because the surrounding space within the bounding box becomes less significant.
If you want to look at the 'innards' of Type 1 fonts there is a free tool, Noah, at http://mywebpage.netscape.com/yeahnoah/index.html (or just search on 'Noah').

Twips?

This term is not used in discussions of typography per se, but may be encountered in DTP documentation.

A twip is one-twentieth of a PostScript point and is derived from 'TWentIeth of a Point'. I don't know where the term originated, but the dimension is used by PageMaker's internals (but is not directly accessible by the user). Visual Basic and Microsoft's Rich Text Format (RTF) use twips; indeed, anyone wanting to program in RTF has to specify screen positions and image sizes in twips.
For ordinary folk the twip is an interesting bit of esoterica, the most useful application of which is to impress others.

Reprinted from the October 2003 issue of PC Update, the magazine of Melbourne PC User Group, Australia

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