The number sign carries a heavier past than its light crosshatch suggests. The leading theory traces it to the abbreviation lb, standing for the Latin libra pondo, meaning pound by weight. Written quickly, again and again by hands in a hurry, the two letters lost their separateness. A stroke was drawn through them to mark the abbreviation, and over time the whole thing collapsed into a crosshatch, the four crossing lines we now recognise. What began as a weight measure, scrawled at speed, hardened into a symbol whose origins its own shape no longer betrays.

The mark's second life came from an unlikely place: the telephone. When Bell Labs engineers were designing the touch-tone phone in the 1960s, they added the symbol to the keypad and then faced a small linguistic problem. The key needed a name, something a person could say aloud, and the existing names felt inadequate. Out of that need emerged octothorpe. The octo is easy to account for, referring to the eight points that stick out around the edges of the mark. The thorpe is another matter entirely, its meaning disputed to this day. One popular claim connects it to the athlete Jim Thorpe, but that is only one story among several, and none has ever been firmly settled.

That lingering uncertainty is part of what makes the character a favourite of the exhibit. Most symbols wear their history plainly once you know where to look; this one keeps a genuine secret, a coined word whose second half nobody can fully explain. The number sign thus lives a curiously divided existence. Its shape descends from a Roman unit of weight, its most colourful name was invented in a telephone laboratory, and part of that name remains a small unsolved puzzle. Whether you call it the number sign, the pound sign in the telephone sense, or the octothorpe, you are reaching back through several centuries and at least one unresolved mystery every time you press the key.