Design factors that improve legibility include the choice of type style, weight, slant and spacing. Letters should be highly legible at small sizes. With larger font sizes, issues of legibility become less of an issue, however readability may suffer. Sans serif faces are more legible than serif faces (especially at smaller sizes). But careful selection of a good sans serif face is important. Some sans serifs work better than others.
Aspects of type design are important is selecting a good typeface for the screen. A relative large x-height improves legibility; especially at small sizes. So do open counter spaces and equal stroke width. Narrow width faces should be avoided as well as extremely extended width faces. Regular to slightly bold faces improve legibility; but care should be taken that boldness does not reduce contrast which is influenced by the amount of counter space. Natscha Frensch (Read Regular for More Effective Reading and Writing 2004), designer of the typeface Read Regular, argues that dyslexic people benefit from fonts that are designed with particular individual distinctions for letters that are similar. For example with ‘b’ and ‘d’ or ‘b’ and ‘p’ she changes the mirroring that traditionally is found in other fonts. We suggest that such fonts might benefit low-vision, older adults and able readers alike.
The spacing of the letters on the screen is equally important to the design of the typeface. Unconsidered, poorly spaced type will detract from legibility. Tracking and interline spacing for type on the screen needs to be larger than for print design.