COM port and RS-232C
An early version of the RS-232 spec defines a 25 pin interface, including a second serial channel with full handshaking, and a few unassigned pins.  It doesn't appear to specify the type of connector to be used, but DB-25 was common.  The early IBM PCs used the DE-9 for the serial port, perhaps due to limited space on the connector panel.

Cannon, who invented the D-subminture connector series, uses the second letter to show the shell size: DA is 15 pin, DB-25, DC-36(or so), DD-50(or so), and DE-9.  (I've used the C and D sizes, but am not certain of the pin count.)