Readability

Readability is a question about a block, not about a character. Nobody reads one letter at a time: the eye moves along a line in short hops, taking three or four words at a stride, and at the end of every line it has to swing back and down to a starting point it has never actually looked at. Everything that makes that journey easy or tiring is the setting rather than the typeface. Size, the width of the column, the space between the lines, the contrast against the background, and how the paragraph is broken.

Measure is the knob with the most published advice attached to it. Something like 45 to 75 characters a line is the range typographers converge on for continuous text, and it is guidance rather than law: it comes from the return sweep, because a line long enough to lose your place on the way back is tiring whatever the face is, and a line short enough to break every three words makes you sweep constantly. Leading is the knob that keeps a long line survivable, which is why the two are set together rather than one at a time, and why the specimen above moves them together. Text balancing and a decent rag deal with the last of it.

Readability is not legibility. Legibility is whether a reader can tell one character from another, which is a property of the drawing; readability is whether a block of text is comfortable to get through, which is a property of the setting. The practical consequence is that they fail separately: a face with a superbly distinct I and l can still be set at 11px across a 110 character line, and a generous, well-led column in a face whose zero and O are the same oval will still cost you a reference number.

Two honest complications. The word is also used, in a different field, for how hard the words are: Flesch and its descendants score sentence length and syllable counts and say nothing about type at all, so an argument about readability can be two people discussing different things. And the last word on the setting is not yours: readers zoom, override fonts, and read at reflow widths you never designed. Contrast is the same story from the other side, which is what APCA is for, and cramming more text onto a screen for density buys space by spending exactly this. Ignoring all of it is a defensible aesthetic position, as brutalist web design shows, but it is a position rather than an oversight.