Originally designed for telephone wiring a modular connector is a type of electrical connector that is used for telephones and variety of other purposes. Some applications that once used larger more expensive connectors were converted to modular connectors. It is likely that the most known use of modular connectors are for telephone and Ethernet jacks.
Once known as registered jacks, modular connectors were originally accepted by the Federal Communications Commission in 1976. The specifications of a registered jack defined the wiring patterns of the jacks instead of the physical dimensions of gender of the connector. They were covered by ISO standard 8877 used in the first ISDN system. Modular connectors have been used to provide low-voltage AC and DC power, but no clear standard exists for this application.
There are other names for modular connectors. Sometimes they are referred to as modular phone jack/plug. They are also known as RJ connector and Western Jack/plug. Modular connectors are called that because their original use was a novel system of cabling that was designed to make telephone equipment more modular.
General Cable Corp developed and patented modular connectors in 1974. They were made to replace the hard-wired connectors that were used on most Western Electric telephones at the time. They replaced screw terminals and bigger 3 and 4 pin telephone jacks that were used in buildings.
Modular connectors do have a gender. Male connectors are known as plugs. Female connectors are known as jacks, or sockets. Plugs are designed to terminate loose cords and cables. Jacks are used in fixed locations most commonly on the wall. It is not common to find a modular connector that has one end with a modular connector and the other end with a jack. Most of the time cables are connected with a male to male adapter that has two female jacks wired back to back.
Modular connectors are made to latch together. They have a spring-loaded tab that is on the plug and snaps into a jack which makes it so the plug cannot be pulled out. Most jacks are installed on a wall or pane with the tab side facing down. This makes it so it is easier to use the tab when your remove the plug.