AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Spacing
Editing, Proofreading, Tagging, and Display
Readability of type depends on the spacing between letters, words, and lines; none of these is independent of the others.
21.7.1 Letterspacing.
Letterspacing refers to the space between letters and other characters. Ideally, the spaces between letters should be balanced. There are no absolute values for optimal letterspacing, but type size and column width are interdependent in design and may affect reading comprehension. Kerning (adjusting the space between characters) is often used to modify spacing between pairs of characters to bring letters closer together or farther apart in an attempt to fit words into a defined space (ie, in text that uses justified columns rather than ragged right line ends). Kerning is typically done in units of 1, 2, or 3. The more kerning units used, the closer the letters become. Kerning should be used cautiously to avoid the merging of letters and reducing legibility.5,7
Examples of changes in the appearance of a line of type that occur by changing the letterspacing are as follows:
21.7.2 Word Spacing.
Typefaces have predetermined spacing between words that is dictated by the point size and width of a typestyle, the darkness or density of the typeface, and the openness or tightness of the letterspacing. For text set ragged right (unjustified), word spacing may be fixed and unchanging. However, for text that is set flush left and flush right (justified), the spacing may need to be more flexible. For justified text, an average word space of a fourth of an em is ideal, with a minimum and maximum range of a fifth of an em to half an em.4
21.7.3 Line Spacing.
Line spacing refers to the vertical distance between the base of 1 line of text and the base of the next line of text. Line spacing is traditionally known as leading for the strips of lead once used between lines of printer type. The space between lines of type is measured in points. Generally, leading is 20% to 45% larger than the copy size.5 For example, 10-point copy would be set on 12 points of leading or line spacing (10/12), as is shown for the body copy in Figure 21.6-1. Optimal line spacing requires consideration of the type size, layout density, and line length. Generally, longer lines call for increased line spacing for optimal readability. See below for different examples of changes in line spacing that change the appearance of the text. More open line spacing also calls for wider margins; tighter line spacing can be done within narrower margins.
No extra line spacing:
The type size and style are identical
in each line; only the space between
the lines changes.
2-Point line spacing:
The type size and style are identical
in each line; only the space between
the lines changes.
4-Point line spacing:
The type size and style are identical
in each line; only the space between
the lines changes.
The conventions for letterspacing and word spacing vary, depending on the amount of spacing between lines, column width and depth, and whether the text is justified (set as a squared-off block) or unjustified (set with a ragged right margin). For example, a smaller type size may be used on a wider column if the line spacing is adequate for readability. The nature of the composed material will suggest whether variations in typography may be effectively used.