LEGIBILITY VERSUS READABILITY
Continuous text needs special attention, as it contains large amounts of detailed information that needs to be easily understood. Readability is concerned with the speed and ease with which the reader can assimilate and retain information printed on the page or screen. Although individual components (letters) may be legible, this does not automatically mean that reading is easy. Readers perceive words not simply as sequences of letters, but as groups of letters and words. These letter and word groupings facilitate speedy recognition as the eye scans the text. Anything that contributes to the breaking up or the slowing down of this scanning process makes for harder and more tiring reading. For example, exaggerated tracking (character and word spacing) will disrupt the normal shape of words (recognizable letter groupings) causing strain to the eye and brain.
Character and word recognition are more easily achieved with upper- and lowercase letters, since they create a greater range of word-icon shapes and individuality than uppercase alone. Capital letters all appear to occupy equal spaces when set in a line of text, so the words they form arc more difficult to discern at a normal reading speed. The relative weight of type can also affect legibility. Medium weights are easiest to read because of the visual balance between the counter-shapes and letterstrokes. Extreme weights of both light and bold type are more difficult and tiring to read as the contrast between the letterstrokes and counter-shapes is distracting.