Legibility

Legibility is a question you ask one character at a time: can this one be told from that one? Capital I, lowercase l and the digit one are three different characters, and in a great many sans-serif faces they are three copies of the same vertical stroke. Capital O and zero are two ovals. An r and an n set close together build most of an m. None of that is a mistake made in the setting. It is in the drawing, which is why legibility travels with the typeface into every size, colour and column width you use it at.

Type designers have a standard kit for pulling those shapes apart: a tail on the lowercase l, serifs top and bottom on the capital I, a flag and a foot on the one, a slash or a dot through the zero, apertures open enough that an e does not fill in and a c does not close into an o, and enough x-height that the lowercase carries its share of the identification. Faces drawn for code and for instrument panels do all of this deliberately, which is why the specimen above sets the same characters in a grotesque and in a monospace face and changes nothing else about them. Optical size is the same argument run down the size range: a drawing that separates cleanly at 34px can close up at 9px.

Legibility is not readability, and the two get swapped constantly. Legibility is whether a reader can tell one character from another; readability is whether a block of text is comfortable to get through, which is a question about the setting (size, measure, leading, contrast). A superbly legible face set at 9px across a 120 character line is unreadable, and a generous reading setting in a face whose I and l are identical will still lose you a serial number.

Two practical notes. It matters most where the reader cannot use context to repair a misread: codes, serials, passwords, part numbers, unfamiliar names. Running prose is self-correcting and a licence plate is not, so the place to audit a face is the confirmation screen, not the marketing headline. And the limiting case of an illegible character is one the font does not contain at all, which arrives as tofu, where no amount of drawing skill elsewhere in the family helps.