I was taught in school that No. was the abbreviation for number. I saw this used everywhere from tables of figures to songs. There’s even a Unicode ligature .

But it only just occurred to me how nonsensical that appears. The word number doesn’t contain the letter O anywhere. Where did it come from?

According to Wikipedia, № is short for the Latin word numero, which means what it sounds like. This has carried through to other romance languages, and eventually to the literary sponge that is English.

The article also states that the pound sign # is more commonly used in North America, which explains a ton of examples in technical documentation that I’ve read of late. I might even prefer it; it’s certainly more succinct.