Leading is the distance from baseline to baseline of two adjacent rows of text. When font designers develop a font, they specify that a given amount of space should appear between rows. The addition of this space ensures that a character is not obscured by part of another character in an adjacent row. There are two ways of adding this additional space: by inserting it within the character cells of a font (internal leading) or by inserting it between rows of text as they are printed on a device (external leading).
Internal leading refers to the space inserted within character cells of a particular font. Only marks such as accents, umlauts, and tildes in foreign character sets appear within the space allocated for internal leading. Figure 2.17 shows two rows of text that use internal leading:
External leading is space inserted between the top and bottom of character cells in adjacent rows of text. The font designer must specify the amount of external leading necessary to produce easily readable text from a particular font. External leading is not built into a font; you must add it before you print text on a device. Figure 2.18 shows external leading: