Before a reader can understand a sentence, their eyes have to resolve its letters, and that lower layer of reading is easy to break without noticing. Thin strokes on low contrast, cramped spacing, decorative typefaces at small sizes: each looks fine in a design review and fails on a phone in sunlight. Legibility is the floor of typography; nothing built above it survives when it gives way.
In this glossary topic:
What is legibility?
Legibility is the ease with which individual characters and words can be recognized in a design. It is a property of the type itself and its immediate setting, how distinguishable an "l" is from a "1", how cleanly letters separate at a given size, and it sits one level below readability, which concerns how comfortably whole passages flow.
What affects legibility?
Four factors do most of the damage or the saving. Typeface choice: letterforms with open shapes, clear distinctions between similar characters, and honest proportions at the intended size. Size itself, since letterforms that sing at 48 pixels can smear at 12. Spacing, where letter, word, and line spacing decide whether characters read as words or as a texture. And contrast between text and background, the factor accessibility standards regulate directly because low-vision readers pay its cost first. The Selecting Typefaces lesson shows how these factors interact in real typeface decisions.
Why does legibility matter?
Illegible text taxes everyone and excludes some entirely: users with low vision, dyslexia, or aging eyes hit the wall first, but every reader pays in speed and fatigue. Interfaces feel "hard to use" for reasons users cannot name, and the cause is often two points of missing contrast or a size chosen on a designer's monitor. Legibility is also one of the few design qualities with regulated minimums, since contrast requirements in accessibility standards are legibility rules wearing legal weight.
