Courier was intended to look like a traditional typewriter font. For many decades, most typewriters didn't have a key for 1 (one), you were supposed to use the

Courier was intended to look like a traditional typewriter font. For many decades, most typewriters didn't have a key for 1 (one), you were supposed to use the lower-case l (ell) instead. Teletypes did have a one key, probably because they were all-uppercase, so there wasn't a lowercase ell available.