Introduction to Arabic numerals: what are old style and monospace numerals?

Arabic (or more accurately, Indian) numerals are single character numerical glyphs like ‘1’ or ‘6’. Perhaps because of ease of access, Arabic numerals are somewhat overused, but they are mandatory for writing out numbers with more than two digits.

It’s not commonly known that numerals come in both capital and lowercase forms. Capital, or lining numerals, are the type most people are most familiar with today, probably because they are almost always the default. Lowercase, or old style numerals, are rarer, but still common in professional typesetting. Some type designers refer to lining and old style numerals as lining and old style figures—this means the exact same thing as numerals.

Numerals also come in different widths—monospace (or tabular) and proportional. Monospace means that all the numbers (glyph plus sidebearings) have the same width; proportional means that they only take up the space they need. This means that a font might include up to four different styles of numerals—monospace lining (usually the default), proportional lining, monospace old style, and proportional old style. Many fonts also include lowercase versions of symbols like the percent sign and currency symbols to go with the old style figures.

A font might include up to four different styles of numerals.
(The font used here is Warnock Pro, by Robert Slimbach).

As you might suspect, capital or lining numerals should be used with capital letters, and lowercase or old style numerals with lowercase letters. Because old style figures are often difficult (or impossible) to access in many apps, it is also acceptable (but not preferable) to set lining numerals in a lowercase context. This website is set with lining numerals—simply because Source Sans, the font I use, doesn’t include old style numerals. However setting old style numerals in a capital context is always wrong.

Think of it this way. Often, we EMPHASIZE certain words by setting them in ALL CAPS. This is WRONG (use italics instead), but widespread, so we usually tolerate it.

BUT WHEN TEXT IS SET IN ALL CAPITALS, (AS IS SOMETIMES STYLISTICALLY DESIRED), IT can LOOK WRONG TO have CERTAIN WORDS in LOWERCASE. IT’S THE SAME deal WITH NUMERALS.

Numeral width exists for a different reason. If prose writers had their way, all numerals would be proportional. They look better that way. You just can’t make the number ‘1’ and the number ‘4’ take up the same space and have it look great. Monospaced numerals only exist for one reason—and that’s for setting tables and rows of numbers. Having all the numbers be the same width makes them line up nicely in a table. So monospaced numerals can be pretty darn useful.

So which numerals should a font include by default? Some fonts, like Scala, put the old style figures as default. But almost all fonts have monospace lining numerals as their default. Why? It’s the safest thing to do. Very few apps let you toggle between the four styles of numbers. So if you are setting tables of numbers, or ALL CAPS text, and your font includes proportional old style numerals as the default, you’re out of luck. Fonts store these characters as glyphs, so it’s possible (but painstakingly difficult) to insert them manually through an Insert Special Character dialog. Their encoding positions are somewhat standardized, at least among fonts from the same foundry, so switching from Warnock to Arno Pro isn’t likely to screw up all your numerals. But this is difficult and risky, so the convention is to place monospace lining numerals in the default slots, since this style is least likely to be wrong.