A keystone coupler is an electrical connector that is used to connect one circuit to another. The female end of a keystone coupler is referred to as a keystone jack, and resembles old-style RJ-11 phone jack plugs on walls where a small rectangular male plug would be inserted into the keystone jack with an attached flexible tab that would snap in place to hold the cable secure. The keystone coupler design, though based on phone line jacks, has since been expanded to a wide range of uses where the wiring can be shielded or unshielded and various numbers of contact points in the jack can be used. Examples of electronics that use a keystone coupler as of 2011 include cordless telephones, fax machines, and computer routers.
One of the most widely-used applications for the keystone coupler as of 2011 is for network cable for Internet or local area network (LAN) service connected to network interface cards (NICs) in personal computers. These keystone coupler parts have a rectangular receptacle that is 14.55 millimeters wide by 16 millimeters high, and are designed to hold category five (cat 5) or category six (cat 6) network cable. Each rating for network cable gauges its ability for handling different levels of transmission frequencies, with cat 5 able to handle up to 100 megahertz and cat 6 up to 250 megahertz.
The overall purpose for using a keystone coupler is that it standardizes the field of electronics so that one device can easily be connected to another, with a high level of data transmission rates between the two. Keystone parts have been adapted to many electronics systems over the past few decades, including fiber optics, audio/video transmissions for television, and speaker terminals for broadcasting music or radio. As well, almost all peripheral devices designed to be connected to computer systems as of 2011 now incorporate some type of keystone coupler into their casing.
The versatility of the keystone coupler also extends to the cabling itself that connects to it. Some couplers have a female keystone jack on both ends so that short runs of networking cable with male keystone plugs can be connected to each side of the coupler to extend the length of the cable. High-definition (HD) television signals also rely on the keystone coupler design for the use of High-Definition Multimedia Interface (HDMI) cabling, which must be used with the HD television to ensure that the quality of the transmission is not degraded.