These are some common metrics used in typography and how they're defined.

Measurement lines on three example typefaces.

Ascender

The part of a lowercase letter that extends above the main body of the letter. English letters with ascenders are b, d, f, h, k, l, and t. Some fonts have very long ascenders and descenders in comparison with the letter bodies, giving them an airy, elegant feel. Fonts with short ascenders and descenders look squatter, more compact, and sturdier.

Ascender line

Invisible line that marks the height of the ascenders in a font.

Baseline

The invisible line on which the bodies of all characters sit. Capital letters are all placed on top of this line, while lowercase letters with descenders will extend below the baseline.

Body

The part of a lowercase letter that does not include ascenders or descenders. In most cases, this is the loop of letters such as b, d, p, and q. A lowercase e is all body, as are a, c, m, n, o, r, s, u, v, w, x, and z. The height of the bodies in a font is called the body height or x height.

Cap height

The height of the capital letterforms. This can be measured as the height of a capital H (not counting serifs). Note that some rounded capitals, such as O, may extend just above the official cap height and just below the baseline. A curve that just touches the cap height (a tangent) is perceived as being a bit shorter than a letter with a flat top.

The letters on the left appear to be the same height, but the left shows the exact same letters with guidelines. Curved letter forms must extend slightly above and below the normal lines to look the same size as more angular letters.

Cap line

An imaginary line demarcating the cap height.

Descender

The part of a lowercase letter that extends below the baseline. English letters with descenders are g, j, p, q, and y.

Descender line

Invisible line that marks the height of the descenders in a font.

Font size

The font size is usually defined as the distance from the descender line to ascender line and is measured in points.

Kerning

Kerning is the spacing between individual letter forms. Modern computer fonts have kerning tables with values for certain letter combinations. For example, TA and LT have upper parts and lower parts that can partially overlap into the neighboring character's space, so they can be brought closer together. Some desktop publishing software allows users to customize kerning to get exactly the effect needed.

	Letters spaced normally
	T-A combination has identical spacing, but seems too disjoint. The T's right edge can safely nest above the A's diagonal stroke.
	Kerned letters. Note that the boundaries actually overlap one another.

Leading

(Pronounced led·ding, not lee·ding) is the vertical distance between the baselines of successive lines of text. This should be a bit more than the height of the font or else ascenders and descenders will be too close (or overlap), causing a cluttered appearance. Double- and single-spacing settings in word processors alter the leading of the text.

x height

The height of the lowercase x character. This measurement gives an idea of how tall lowercase letters are and how large the bodies are of characters with ascenders and descenders. Also called body height.