# Rules for Typesetting Diagonal Fractions

Fractions are an essential element in typesetting. They appear with great regularity in measurements and quantities, recipes, scores, numerical charts and listings, textbooks and manuals, as well as math and science.

Diagonal fractions are the most commonly-used fraction style in typeset copy. When they are called for, the challenge lies in knowing which diagonal fractions are available in any given font, and how to access or create them. Here’s some tips and general rules for typesetting diagonal fonts.

Diagonal fractions fall into three categories: *basic, extended*, and* arbitrary*.

*Basic* fractions are the ones that are available in most fonts regardless of the font format, and usually compromise ¼, ½, and ¾.

*Extended *fractions are a broader collection of diagonal fonts that are found in many – but not all – OpenType fonts. They consist of 1/8, 3/8, 5/8, 7/8, and sometimes 1/3 and 2/3. Extended fractions are also referred to as *prebuilt *fractions, as they are part of a font’s character complement.

*Arbitrary *fractions are any other random fraction, such as 25/1000 and 76/389. They are created on-the-fly by the fraction-building function available in some OpenType fonts, and therefore are not actual glyphs found in a font.

**Setting Basic and Extended Fractions**

The first step is finding out which fractions are available in any particular font. Both basic and extended fractions can be found in the Glyphs panel of most design software, usually under the Numbers subset.

Once you determine that a font has the *prebuilt* fractions you need, there is an easier way to set these fractions that does not require accessing them from the Glyphs panel.

Here’s how to access *basic and extended fractions* in InDesign:

- Set the fraction manually, that is, using full sized numbers separated by a slash
- Highlight this horizontal fraction
- Then select Fractions via InDesign’s Character panel > OpenType panel (or submenu)

If using Illustrator, the process is slightly different:

- Set the fraction manually
- Highlight the horizontal fraction
- Then click on the Fractions icon in the OpenType panel

(Note that Illustrator has a separate OpenType panel vs. InDesign’s OpenType submenu accessed off of the Character panel.)

In both cases, the horizontal fraction will automatically be replaced with the prebuilt, diagonal style, if available in that font.

**Setting Arbitrary Fractions **

Arbitrary fractions are built on-the-fly using superior and inferior figures as well as a fraction bar, and therefore are not located on the Glyphs panel. As previously mentioned, the ability to create arbitrary fractions on-the-fly is a feature that is programmed into some, but not all, OpenType fonts.

The bad news is there is no easy way to determine which OpenType fonts have this ability; it is mostly a matter of trial and error to determine which do and which don’t. But the good news is there are still plenty of great-looking fonts that do, especially those intended for text.

The steps to setting* arbitrary* fractions are the same as prebuilt ones:

- Set the fraction manually
- Highlight the horizontal fraction
- Then select Fractions via InDesign’s Character panel > OpenType submenu, or Illustrator’s Fractions icon.

If the font in question has the ability to set arbitrary fractions, it will automatically be converted. If not, nothing will happen.

**Note:** Only use the Fraction command for individual fractions, and not for an entire block of text, or all full-sized numerals will be converted to superscript/superior or subscript/inferior figures.

Other fraction styles include nut or vertical fractions (often used for math and science formulas), horizontal fractions (considered unattractive and unprofessional in most typesetting), decimal (5.25, etc.), and spelled out. Make sure you do your research, and use the appropriate fraction style for the job at hand.