Apart from composition, readability also covers vocabulary difficulty, sentence structure and abstractness of relationships expressed by the words. Such aspects are concerned with expressing precisely what one means in an unambiguous manner. These aspects are, of course, the responsibility of the author rather than the typographer and will therefore not be discussed here. The complex of aspects mentioned here is also often called reading ease instead of readability. A detailed review of the scientific studies and investigations relating to reading ease is provided by Zakaluk and Samuels (1988).

As is the case for legibility, readability is also measured in several ways. In addition to the development of indices of reading ease, which are not related to typography by nature, studies of reading speed, wordsearch speed, eye-movement, reader fatigue, and comprehensibility have been undertaken. However, most research literature does not state whether its topic is legibility or readability measurement. In fact, the terms legibility and readability cause some confusion because the two concepts are defined in many different ways. The reason for this is that legibility was used in a more general sense at the beginning of typographic research in the 1890’s. Only when the term readability was introduced in the 1930’s did the first term come to express the concept which is nowadays called legibility. The confusion, still existing among some designers and even researchers, is upheld by the fact that designers seem not to make the distinction between both terms (Søgren, 1995). Perhaps some typographers consider the distinction to be of little practical use. As a result, the terms are often used interchangeably, so care should be taken when reading any text that contains either of these terms.