By the twentieth century the # had a settled life in print as the pound and number sign. Then the telephone gave it a brand-new, deliberately silly name that still confuses people today: the octothorpe.
Two spare keys on the touch-tone pad
When Bell Labs designed the touch-tone (DTMF) telephone keypad in the 1960s, the ten digits left two empty corners on the bottom row. Engineers filled them with two symbols already familiar from the typewriter, the asterisk (*) and the number sign (#), to give the phone network extra signaling characters for features that had not been invented yet.
That left a small but real problem. The company needed a single, unambiguous name for the # key so that documentation and training would not descend into an argument. “Pound,” “number sign,” and “hash” were all already taken, and all meant different things to different people. So, as the story goes, Bell Labs engineers simply invented a new word.
Octo- plus -thorpe
They called it the “octothorpe.” The “octo-” part is the easy half: the symbol has eight points around its outside. The “-thorpe” part is where it gets fun, because nobody fully agrees on it, and the people who were there told slightly different versions.
One account, from Bell Labs engineer Doug Kerr, credits colleagues who wanted a name no two people could confuse, and traces the ending to an in-joke. Other tellings tie “thorpe” to the Olympic athlete Jim Thorpe, to the old English word “thorp” meaning a small village or hamlet (with the symbol read as eight fields around a central village), or simply to a sound that was odd enough to be memorable. The word even shows up spelled several ways in early documents: octothorp, octalthorpe, octatherp.
A name built to be unmistakable
The oddness was the point. A made-up word could not be confused with “pound” (weight or money), “number,” “hash,” or the musical “sharp,” so it did exactly the job Bell Labs needed: one key, one name, no debate. The word never quite caught on with the public, who kept saying “pound” or “hash,” but it stuck as the formal name. To this day, “octothorpe” is the answer to the trivia question of what the # symbol is really called.