From the article: >Standard RS-232 as defined in 1962 used a roughly D-shaped shell with 25 physical pins (DB-25), way more than the physical protocol actually

From the article: >Standard RS-232 as defined in 1962 used a roughly D-shaped shell with 25 physical pins (DB-25), way more than the physical protocol actually required (you can support a minimal version with just three wires, and this was actually common). Twenty years later, after the IBM PC-AT introduced it in 1984, most manufacturers switched to using a smaller DB-9 connector (which is technically a DE-9 but almost nobody ever called it that) >Almost nobody ever called it that