I believe the "D" refers to it's shape. The "9" is the number of possible pin positions. There were several variations of this but I believe all configurations had an odd number of pins. I recall a transmission protocol named 20 mill current loop and later there was RS-232 (serial) that used the same plugs and jacks but devices not matching in protocol were not compatible with one another. The first serial mice were RS-232. That later gave way to PS-2 use for mice and keyboards. Serial outputs on older PCs were usually "D" connectors in 9, 15, 21, and 25 pin configurations. (someone will correct me if I'm wrong but the 9 and 21 pin connectors were for serial ports 1 and 2 and the 25 pin port was for printers using the Centronics parallel standards. The 15 pin port was for VGA. That one is still around but fading.