I always thought that the COM port on a classic PC implements the RS-232C serial interface. But I just noticed that the COM port is a DB-25, whereas RS-232C only uses 8 or 9 ports, and therefore tends to use DE-9. Does the DB-25 serial port implement RS-232C, or does it implement some other serial interface, and if so, what standard is this interface?
All this question requires is a simple Google search to understand the original standards in place.
Cannon's part-numbering system uses D as the prefix for the whole series, followed by one of A, B, C, D, or E denoting the shell size, followed by the number of pins or sockets, followed by either P (plug or pins) or S (socket) denoting the gender of the part.
The second letter (A, B, C, D, E) did not denote the number of connections, but the shell size. There was single, high and double density configurations in all shell sizes.
So on a PC you would see DB-25P or DE-9P connectors for RS232 and you see a DB-25S used for the parallel port.
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.)