Basic Book Design/Leading

Keep Out of Trouble Rules

When using book-length lines (65-70 characters per line, including spaces), don't use single-spaced or double-spaced settings. Instead, use "At Least" settings.
When using book-length lines, use two or three points of leading, e.g., 15 points leading with a 12-point font.
When using wider lines, e.g., in a letter, use wider leading. Use double-spacing or one-and-a-half spacing.

Definitions

leading
solid

Leading

Leading (pronounced like lead, the metal, not lead, what your dog does when walking you) is the space between lines of text. It's also called spacing, as in single-spaced, double-spaced, etc.

Fonts and leading are referred to as two numbers. E.g., "12/13" or "12 on 13 points" is a 12-point font with 13 points of leading.

Solid

Single-spaced can be the same as the font size (called solid), e.g., a 12-point font on 12-point leading. Or it can be the font size plus one point, e.g., a 12-point font on 13 points of leading is single-spaced. Check what your word processor does.

A 12-point font with one-and-a-half spacing is 18 points of leading, or 12/18.

A 12-point font with double spacing is 24 points of leading, or 12/24. Use two or three points of leading. E.g., 11/13, or 12/15. This is easier to read than single-spaced (one point of leading).

More than three points of leading is usually unnecessary. Your book will be longer, increasing printing costs. Readers will have to move their eyes more, and turn more pages.