Typographical features
There are many elements in the design of a typeface which can contribute to its legibility.
Serif / Sans Serif
"Serifs" are the small finishing strokes on the end of a character. "Sans serif" fonts do not have these small finishing strokes.
Figure 1
Examples of serif and sans serif letters
Point size
Point size is perhaps the element most used to describe the legibility of a type face, but it can also be the most deceptive. Point size is a legacy from the letterpress system, where each letter is held on a small metal block. The point size actually refers to the size of this metal block, and not the actual size of the letter. The letter does not have to take up the full area of the block face, so two fonts with the same nominal point size can quite easily have different actual sizes. ( Bix, 2002)
Figure 2
The difference between point size and actual letter size (Image © Bix, 2002 )
X-height
X-height refers to the height of the lower case "x" in a typeface. It is often a better indicator of the apparent size of a typeface than point size ( Poulton, 1972 ; Bix, 2002 ).
Figure 3
X-height
Counters
Counters are the "negative spaces" inside a character. They are also good indicators of the actual size of the type.
Figure 4
Counters
Ascenders and descenders
Ascenders and are the vertical strokes which rise above the body of a character or x-height. Descenders are strokes which fall below the baseline of the x-height.