Legibility is the degree to which glyphs (individual characters) in text are understandable or recognizable based on appearance. “The legibility of a typeface is related to the characteristics inherent in its design ... which relate to the ability to distinguish one letter from the other.” Legibility includes factors such as “x-height, character shapes, stroke contrast, the size of its counters, serifs or lack thereof, and weight.”
Factors such as x-height, counter size, letter spacing and stroke width which are more significant for legibility than the presence or absence of serifs.
This serif font is Georgia 1lI rn jg
This serif font is Times New Roman 1lI rn jg
This sans-serif font is Verdana 1lI rn jg
This sans-serif font is Arial 1lI rn jg
Here are some letters and a number – nhy jg pbd 1lI rnm – set in Arial, Lucida, Verdana and Optima which can be hard to distinguish from one another because the look alike, run into each other and look like another letter, or look very similar when they are reversed or rotated for those with dyslexia.