In typography, the baseline is the imaginary line upon which a line of text rests. In most typefaces, the descenders on characters such as g or p extend down below the baseline while curved letters such as c or o extend ever-so-slightly below the baseline.
The baseline is the point from which other elements of type are measured including x-height, cap height, and leading. The baseline is also significant in the alignment of drop caps and other page elements.
Examples: The amount of space between the baselines of two lines of type — the leading — combined with how far below the baseline that descenders go (and how high ascenders go), can affect the readability of the text.