One could say that legibility is a document designer’s main goal. Legible type is large enough and distinct enough that a reader is able to discriminate individual words and/or letters. Text may be legible but not readable if the reader is unable to read easily and smoothly and quickly becomes tired and bored. Readability is therefore the quality that makes the page easy to read, inviting, and pleasurable to the eye. Readability, not legibility, is, in fact, the document designer's goal.

When considering readability, document designers must take into consideration the design of individual faces and the subtleties of word and character spacing, as well as the nature of the material and the subjective circumstances of reading.

The major factors affecting readability relate to the relative proportions of horizontal to vertical space: line width, type size, and spaces between lines, words, and letters.

Studies have shown that when we read, our eyes go through a series of jerky motions called a saccadic movement, punctuated by pauses called “fixation pauses.” We read a group of words within one eye span and then pause and shift our eyes, to another group of words. At normal reading distance, an eye span is about 13 picas wide. If a column of text type is set too wide slightly more than two spanswe move our heads as well as our eyes. This makes for both tiring and inefficient reading. The upper limit, then, for the length of a line meant for continuous reading is about 27 picas.

We also go through a process of rereading that is called “regression,” and, when we return our eyes from one line of type to the next, it is called “return sweep.” We often reread material, usually key words, in previous lines; how often we do this depends partly on the complexity of the material. This rereading process is thought to aid short term memory as we continually make connections between what has just been read and what is currently being read. Too long a line makes it harder for the reader to find the beginning of the next line, creating confusion and difficult reading.

Many factors affect readability although the two main elements that make for good and readable composition are the correct proportions of type size to line (or “column”) width, and the horizontal flow created by the white space between the lines. Type that is too large for the column width limits the number of words that will fit on a line, creating uneven word spacing and excess hyphenation. Type that is too small for the column, forces the reader to move closer to the page, effectively reducing the eye span and increasing the number of saccadic movements to the end of the line. This quickly leads to fatigue.

The proportion and distribution of white space between lines, words, and characters are also major factors in determining readability. There must be enough space between the lines so that the eye can easily move in a horizontal direction, enough space between words so that they can be perceived as units, and enough space between letters so that they can be distinguished. On the other hand, there must not be so much space that the eye fails to easily make the transition from the end of one line to the beginning of the next, so that words do not flow easily into each other, or so that letters do not compose visually into words.

Rather than reading individual letters, according to some studies, we read words and groups of words, and recognition depends a great deal on what we have become used to. An example of this is our ability to read words that are set in upper and lower case, even if the bottom half of a word is missing. We recognize the shape of the word, not the individual letters. This is why it is not a good idea to set text type in capital letters: words appear to us as horizontally-oriented rectangles, and we are forced to stop the flow of our reading to decipher words in all caps. Text made up of typefaces with very large x-heights tends to exhibit some of the same problems as words that lack the visual outlines we are used to seeing.

Since serif faces tend to move the eye along the horizontal direction of reading, the serifs themselves become an additional means of differentiating letters from one another. It is widely assumed that serif typefaces are more legible than sans serif ones. It has not been conclusively shown that sans serifs decrease legibility, yet many people do find that in continuous reading sans serif type can be tiring.

The key relationships for readability are, therefore, those that exist between visual type size (characters per pica), line length, and line spacing.