This is the pin connections and diagram for creating a RS-232 "null modem", which is used to communicate between two computers, with a modem.
Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using a RS-232 serial cable. The original RS-232 standard only defined the connection of DTEs with DCEs i.e. modems. With a null modem connection the transmit and receive lines are crosslinked. Depending on the purpose, sometimes also one or more handshake lines are crosslinked. Several wiring layouts are in use because the null modem connection is not covered by a standard.