The visual clarity of text, generally based on the size, typeface, contrast, text block, and spacing of the characters used.

Confusion regarding the research on legibility is as persistent as it is pervasive. The rapid growth and advancement of modern desktop publishing, Web-based publishing, and multimedia presentation continue to compound the confusion with increasing font and layout capabilities, display and print options, and the need to effectively integrate with other media. The following guidelines address common issues regarding text legibility.1

Size

For printed text, standard 9- to 12-point type is considered optimal. Smaller sizes are acceptable when limited to captions and notes. Use larger type for low-resolution displays and more senior audiences.2

Typeface

There is no performance difference between serif and sans serif typefaces, so select based on aesthetic preference. Sentence case text should be used for text blocks. On low-resolution displays, antialiasing the text may marginally improve legibility, but primarily serves as an aesthetic enhancement of the typeface.3

Contrast

Use dark text on a light background or vice versa. Performance is optimal when contrast levels between text and background exceed 70 percent. Foreground/background color combinations generally do not affect legibility as long as you observe the minimum contrast level, so select based on aesthetic preference. Patterned or textured backgrounds can dramatically reduce legibility, and should be avoided.4

Text Blocks

There is no performance difference between justified and unjustified text, so select based on aesthetic preference. For 9- to 12-point type, a line length of 3 to 5 inches (8 cm to 13 cm) is recommended, resulting in a maximum of about 10 to 12 words per line, or 35 to 55 characters per line.5

Spacing

For 9- to 12-point type, set leading (spacing between text lines, measured from baseline to baseline) to the type size plus 1 to 4 points. Proportionally spaced typefaces are preferred over monospaced.

See also Iconic Representation and Readability.

SIZE

TYPEFACE

Serif vs. San Serif Serif typefaces have small “feet” at the ends of the letters.

Antialiased vs. Aliased Text Antialiased text looks smooth because of pixels added to smooth the transition between the text color and the background color. Aliased text looks jagged because it does not contain these transition pixels.

Text Cases

Uppercase vs. Mixed Case People recognize words by letter groups and shapes. Uppercase text is more difficult to read than sentence case and title case because the shapes of uppercase words are all rectangular.

CONTRAST

TEXTBLOCKS

Aligned Left, Ragged Right Text

Justified Text

Aligned Right, Ragged Left Text

SPACING

Leading
Leading (rhymes with sledding) is the amount of vertical space from the baseline of one line of text to the baseline of the next line of text. Below, the type size is 12 points and the leading is 18 points.

Monospaced vs. Proportionally Spaced Typefaces In monospaced typefaces, all characters assume the same amount of horizontal space. In proportionally spaced typefaces, characters assume variable amounts of horizontal space, depending on the width of the actual character and the relationships among groups of characters.