DIGITAL TYPOGRAPHY & GRAPHICS
ARTICLES ON SEPARATE PAGES:
CONTROLLING TEXT AESTHETICS WITH KERNING AND TRACKING
posted: January 16, 2016
ARTICLES ON THIS PAGE:
“UNITS”: A FUNDAMENTAL MEASURE OF SPACE AND WIDTH
updated November 9, 2016; posted September 31, 2016
SPECIAL SPACES IN UNICODE AND INDESIGN
posted September 31, 2016
“UNITS”: A FUNDAMENTAL MEASURE
The term “unit,” as a measure of space or width, is important for fine work in digital type. A unit is always a fraction of an em. (An em is a basic typographic proportional measurement: a square with height and width equal to the point size. Thus, for 18-point type, an em is 18 points wide.)
OF SPACE AND WIDTH
Which fraction of an em depends on the particular technology, or the particular software, you are using.
The unit is not a fixed value in any system; its value is relative to the value of an em at whatever point size you are using at the moment. In other words, the unit is a relative measure, not an absolute one. It is relative to the point size, which determines the value of an em. For any given unit system you can find the value of a unit in points, at a given point size, with the following formula: point size divided by the number of units in an em.
The first use of a unit system in typesetting was in the hot-metal days, in the Monotype Composition Caster system, which used a unit of one eighteenth of an em.
Eighteen was a magic number for typography in the twentieth century. A system based on eighteen, or one of its multiples, yields mainly numbers that are divisible by two or three, or by their multiples. I would guess that it was this that made it feasible to build an analog computer—that is, a mechanical computer like the Monotype Composition Caster system—that would function for setting type.
Perhaps more importantly, a base-18 system made it easier to do the needed calculations with a proportion wheel, a hand calculator, or in your head. Some of us did such calculations very, very frequently. (In the 80s, I had a table of unit values for 54ths of an em on a pencil holder on my desk.)
In the 80s and 90s, at least, pre-desktop typesetting systems that ran on digital computers used units of 18ths, 36ths, 54ths, or 108ths of an em. 54ths was the most common. It seemed, at the time, quite adequate for fine work.
Desktop typesetting software broke away from the 18-based systems. (I’d guess that increased processing power made this possible.)
Quark used units of one two-hundredth of an em. I found this to be a very handy unit value, not too coarse and not too fine, so that most kerning and tracking adjustments in running text required only one to three units—and one to three keystrokes.
Quark’s units could be further divided into smaller decimal fractions, but I rarely found occasion to do so: only for fine kerning of very large display type, or for tracking very small legal type on long measures, to lose a line without tightening the already hard-to-read type any more than was absolutely necessary.
Fontographer, long a standard desktop application for font creation and modification, used units of one thousandth of an em. So did (and does) Illustrator. InDesign followed this.
Such a fine unit is probably useful for font creation and font editing, but it is needlessly fine for even the finest typesetting. With InDesign, I set the Units & Increments preference so that the Kerning/Tracking unit (the amount added or subtracted by keying Option-Left/Right Arrow) is five thousandths of an em—which is equal to one two-hundredth of an em. InDesign’s default keystroke value is 20 thousandths of an em, which is much too coarse—so much so that it probably deters people from working with units. (When reading or inputting numerical values for units in InDesign, you are still working with thousandths.)
For more on fine work with kerning and tracking in InDesign, see the article Controlling Text Aesthetics with Kerning and Tracking.
SPECIAL SPACES IN UNICODE AND INDESIGN
(Not a full treatment of the subject, but some useful notes.)
Unicode has a thin space (U+2009) that is one fifth or one sixth of an em, and a hair space (U+200A) that is narrower than a thin space. These can vary with the application used to set the type, and perhaps also with the font, at the option of the designer. (Unicode also has a 3-, 4-, and 6-to-em space, en and em quads, as well as a punctuation space, and a figure space distinct from the en quad.) The hair space, thin space, and some of the others are not meant for typographic use, but only to facilitate some conversions
InDesign has a hair space and a thin space, in addition to sixth-, quarter-, and third-spaces, and also punctuation, em, en, and figure spaces. (Quark, as I recall, had something similar.) The thin space and hair space are said to be breaking in Unicode, but non-breaking in InDesign.
The punctuation space could be useful in some types of work, especially tabular work, since the punctuation space would take up the space of a period or comma between divisions of a large number. The punctuation space is said to be breaking in Unicode, but non-breaking in InDesign.
I don’t know if, in InDesign, the hair, thin, punctuation, and figure spaces yield fixed widths regardless of font. (See the paragraph on Unicode, above.)
A quick test, measuring paper output, with OpenType and non-OpenType versions of 12-point Trump Mediaeval gave the following approximate values, the same for both OpenType and non-OpenType fonts: hair space: about one sixtieth of an em; thin space: about one thirteenth of an em; punctuation space: about one-quarter of an em; figure space; about one twenty-fourth of an em larger than the en space.
I have seen it written that InDesign’s hair and thin spaces are one twenty-fourth and one eighth of an em, respectively.
I have used InDesign’s thin space occasionally for setting French, as the space between some punctuation marks and the word they precede or follow.
Otherwise, I have never had any real use for the hair, thin, punctuation, and figure spaces–even though I’ve done a great deal of financial work. They are probably useful only in shops where certain types of specialized work are done regularly, by people who are familiar with any tradeoffs their use may involve. I’ve always used en spaces for figure spaces.
The thin and hair spaces might be useful for tweaking the spacing between certain character pairs—you could do a search-and-replace to apply the same adjustment throughout a long text. However, the presence of the spaces might cause problems if the same string was later included in another search-and-replace string, or if the text was moved to another application. I’ve always used hand kerning (Option-left/right arrows) for such adjustments—or, if there was a lot of work where the same adjustment was required, I’ve changed the kerning metrics in the font.
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