A hair's breadth, or the width of human hair, is used as an informal unit of a very short length. It connotes "a very small margin" or the narrowest degree in many contexts. Until the middle of the 20th century, the highest resolution of measurement was considered to be the same order of magnitude, around 10−5 metres, as the diameter of a human hair. A "hair's breadth" was, and still is, informally, a very small measurement.