RJ11 jacks are widely used in communication equipment such as telephones and fax machines. As 4‑pin or 6‑pin connectors, they are collectively referred to as RJ11 in the industry.

RJ45 jacks are generally applied for data transmission, most commonly for network adapter and network switch interfaces. These are 8‑pin connectors, known universally in the industry as RJ45. There are two standard wiring schemes for RJ45 crystal plugs: crossover cables for connections between devices of the same type, and straight‑through cables for connections between different types of devices.