EIA-232 specifies the gender and pin use of connectors, but not their physical type. RS-423 specifies the electrical signals. 25-way D-type connectors are common but often only three wires are connected - one ground (pin 7) and one for data in each direction. The other pins are primarily related to hardware handshaking between sender and receiver and to carrier detection on modems, inoperative circuits, busy conditions etc.
The standard classifies equipment as either Data Communications Equipment (DCE) or Data Terminal Equipment (DTE). DTE receives data on pin 3 and transmits on pin 2 (TD). A DCE EIA-232 interface has a female connector. DCE receives data from DTE on pin 2 (TD) and sends that data out the analog line. Data received from the analog line is sent by the DCE on pin 3(RD).
Originally DCE was a modem and DTE was a computer or terminal. The terminal or computer was connected (via EIA-232) to two modems, which were connected via a telephone line.
The above arrangement allows a computer or terminal to be connected to a modem with a straight-through (2-2, 3-3) cable. It is common, however, to find equipment with the wrong sex connector or with pins two and three reversed, requiring the insertion of a cable or adaptor wired as a gender mender or null modem. Such an adaptor is also required when connecting a computer directly to a terminal or to another computer without the use of modems.