Hollerith code
The character encoding on a punched card: twelve positions per column, where one hole is a digit and two holes are a letter — and whose gaps still show up in EBCDIC.
Every column on an 80-column card has twelve punchable positions: rows 0–9 plus two extra rows above them, called 11 and 12. A single hole in rows 0–9 means that digit. A letter takes two holes — a zone row (12, 11 or 0) plus a digit 1–9. So 12+1 is A, 12+9 is I, 11+1 is J, 0+2 is S. Special characters used a third punch, usually in row 8. The encoding is not arbitrary; it follows what a punch die can plausibly do to a piece of card stock without shredding it.
Hollerith's own 1890 census cards used round holes and a different layout; the rectangular 80-column card arrived with IBM in 1928 and the code grew in stages after that. The inheritance is clearest in EBCDIC, which IBM built in 1964 straight on top of the zone/digit split: the alphabet is not contiguous but arrives in three clumps, A–I, J–R and S–Z, with gaps between them. Programmers have cursed those gaps for sixty years, not always aware they are cursing the geometry of a piece of cardboard.