The Versatile Octothorpe

By Lucy Ferriss. Hashtag. Pound sign. Space. And then there’s the use of the octothorpe to replace the word number, as in “He was #4 in the queue.” How did those little cross-hatched horizontal and vertical lines come to mean so many different things?
Apparently it begins, like so much else, with the Romans, whose abbreviation for libra pondo, or pound weight, was (as it still is) lb, but with a stylized l including a finishing, cursive-like slash across the center, to distinguish it from the number 1. By 1850, bookkeepers had adopted two uses of the octothorpe: If it followed a number, it retained the sense of pounds, but if it preceded a number, it simply indicated number. More...