Principal line terms in typography

## Baseline

In European and West Asian typography and penmanship, the baseline is the line upon which most letters sit and below which descenders extend.

In the example to the right, the letter 'p' has a descender; the other letters sit on the (red) baseline.

Most, though not all, typefaces are similar in the following ways as regards the baseline:

* Capital letters sit on the baseline. The most common exceptions are the J and Q.
* Lining figures (see Arabic numerals) sit on the baseline.
* The following text figures have descenders: 3 4 5 7 9.
* The following lowercase letters have descenders: g j p q y.
* Glyphs with rounded lower and upper extents (0 3 6 8 c C G J o O Q) dip very slightly below the baseline ("overshoot") to create the optical illusion that they sit on the baseline, and rise above the x-height or capital height to create the illusion that they have the same height as flat glyphs (such as those for H x X 1 5 7). Peter Karow's Digital Typefaces suggests that typical overshoot is about 1.5%.

The vertical distance of the base lines of consecutive lines in a paragraph is also known as line height or leading, although the latter can also refer to the baseline distance minus the font size.

Northern Brahmic scripts have a characteristic hanging baseline; the letters are aligned to the top of the writing line, marked by an overbar, with diacritics extending above the baseline.

East Asian scripts have no baseline; each glyph sits in a square box, with neither ascenders nor descenders. When mixed with scripts with a low baseline, East Asian characters should be set so that the bottom of the character is between the baseline and the descender height.

## Descenders

In typography and handwriting, a descender is the portion of a letter that extends below the baseline of a font.

For example, in the letter y, the descender is the "tail", or that portion of the diagonal line which lies below the v created by the two lines converging. In the letter p, it is the stem reaching down past the o.

In most fonts, descenders are reserved for lowercase characters such as g, j, q, p, y, and sometimes f. Some fonts, however, also use descenders for some numerals (typically 3, 4, 5, 7, and 9). Such numerals are called old-style numerals. (Some italic fonts, such as Computer Modern italic, put a descender on the numeral 4 but not on any other numerals. Such fonts are not considered old-style.) Some fonts also use descenders for the tails on a few uppercase letters such as J and Q.

The parts of characters that extend above the x-height of a font are called ascenders.

Descenders are often reduced in small-print typefaces for uses such as newspapers, directories or pocket Bibles to fit more text on a page. More radically, on 20 May 1802 Philip Rusher of Banbury patented a new Patent Type with eliminated descenders and shortened ascenders. The type did not prove successful, nor did another use in 1852. The Art Nouveau display typeface Hobo and headline typeface Permanent Headline which also eliminate descenders have both been somewhat popular since.

Some early computer displays (for example, the Compukit UK101) and printers (for example, the Commodore 4022) restricted the vertical spacing of characters so that there was no space for correct display of descenders. Instead, characters with descenders were displaced vertically upwards so that the bottom of the descender was aligned with the baseline. Contemporary systems that did not have this restriction were described as supporting true descenders.

## Ascenders

In typography and handwriting, an ascender is the portion of a minuscule letter in a Latin-derived alphabet that extends above the mean line of a font. That is, the part of a lower-case letter that is taller than the font's x-height.

Ascenders, together with descenders, increase the recognizability of words. For this reason, many situations that require high legibility such as road signs avoid using solely capital letters (i.e. all-caps).

Studies made at the start of the construction of the British motorway network concluded that words with mixed-case letters were much easier to read than "all-caps" and a special font was designed for motorway signs. These then became universal across the UK.

In many fonts intended for body text, such as Bembo and Garamond, ascenders rise above the cap height of the capital letters.

## X-Height

In typography, the x-height, or corpus size, is the distance between the baseline and the mean line of lowercase letters in a typeface. Typically, this is the height of the letter x in the font (the source of the term), as well as the letters v, w, and z. (Curved letters such as a, c, e, m, n, o, r, s, and u tend to exceed the x-height slightly, due to overshoot; i has a dot that tends to go above x-height.) One of the most important dimensions of a font, x-height defines how high lowercase letters without ascenders are compared to the cap height of uppercase letters.

Display typefaces intended to be used at large sizes, such as on signs and posters, vary in x-height. Many have high x-heights to be read clearly from a distance. This, though, is not universal: some display typefaces such as Cochin and Koch-Antiqua intended for publicity uses have low x-heights, to give them a more elegant, delicate appearance, a mannerism that was particularly common in the early twentieth century. Many sans-serif designs that are intended for display text have high x-heights, such as Helvetica or, more extremely, Impact.

### Design Considerations

Medium x-heights are found on fonts intended for body text, allowing more balance and contrast between upper- and lowercase letters and a brighter page. They then increase again for optical sizes of fonts designed for small print, such as captions, so that they can be clearly read printed small.

High x-heights on display typefaces were particularly common in designs in the 1960s and '70s, when International Typeface Corporation released popular variations of older designs with boosted x-heights; notable examples of this trend include Avant Garde Gothic and ITC Garamond. More recently, some typefaces such as Mrs Eaves, Neutraface and Brandon Grotesque have been issued with distinctively low x-heights to try to create a more elegant appearance. While computers allow fonts to be printed at any size, professional font designers such as Adobe issue fonts in a range of optical sizes optimized to be printed at different sizes. As an example of this, Mrs Eaves exists in two versions: an original style intended to give an elegant, bright appearance, and a less distinctive 'XL' design intended for body text.

Some research has suggested that while higher x-heights may help with reading smaller text, a very high x-height may be counterproductive, possibly because it becomes harder to identify the shape of a word if every letter is nearly the same height. For the same reason, some sign manuals discourage all-capitals text.

### Use in web design

In computing one use of x-height is as a unit of measurement in web pages. In CSS and LaTeX the x-height is called an ex. The use of ex in dimensioning objects, however, is less stable than use of the em across browsers. Internet Explorer, for example, dimensions ex at exactly one half of em, whereas Mozilla Firefox dimensions ex closer to the actual x-height of the font, rounded relative to the font's current pixel height. Thus, the exact ratio of ex to em can vary by font size within a browser if the determined values are rounded to the nearest whole unit. For example, a browser calculating an x-height of 45% on a font 10 pixels tall may round ex to either 4 pixels or 5 pixels or leave it at 4.5 pixels.

### Other important dimensions

Lowercase letters whose height is greater than the x-height either have descenders which extend below the baseline, such as y, g, q, and p, or have ascenders which extend above the x-height, such as l, k, b, and d. The ratio of the x-height to the body height is one of the major characteristics that defines the appearance of a typeface. The height of the capital letters is referred to as cap height. x-height is most important in regular designs, such as most serif and sans-serif designs; script typefaces that mimic irregular handwriting and calligraphy may not have a consistent x-height across all letters.

## Mean line

In typography, the mean line is the imaginary line at the top of the x-height.

Round glyphs will tend to break (overshoot) the mean line slightly in many typefaces, since this is aesthetically more pleasing, otherwise curved letters such as a, c, e, m, n, o, r, s, and u will appear visually smaller than flat-topped (or bottomed) characters of equal height, due to an optical illusion.

## Cap height

In typography, cap height is the height of a capital letter above the baseline for a particular typeface. It specifically is the height of capital letters that are flat—such as H or I—as opposed to round letters such as O, or pointed letters like A, both of which may display overshoot. The height of the small letters is the x-height.