A distinction needs to be made between character legibility and legibility of printed text. Character legibility is the ease with which a person can identify an individual character as a particular letter. Legibility of text refers to the ease with which groups of characters are correctly identified as a word, with the result that the reader perceives meaningful sentences.

High legibility is very important for books and other reading-intensive print, perhaps more so than for logotype or a poster. If a text is not very legible, this will grossly affect the speed with which the text is read and will increase the mental effort needed to identify the letters correctly. Legibility is affected by level of illumination, background contrast and reader fatigue.A number of different measures have been used to determine the (level of) legibility of typefaces and styles of type. The kind of type that allows the highest reading speed has often been judged to be the most legible. Another measure of legibility is the maximum distance from the eye that allows accurate identification of letters. Other measures that have been used are: the number of errors of recognition in short time exposure of text and the rate of eye-blinking during reading. Also, several indices of legibility have been proposed as a measure to express the recognizability different kinds of type. Examples are the legibility coefficient and specific legibility. A review of these methods and formulae can be found in Zachrisson (1965).