A 19-inch rack is a standardized frame or enclosure for mounting multiple electronic equipment modules. Each module has a front panel that is 19 inches (482.6 mm) wide. The 19 inch dimension includes the edges or ears that protrude from each side of the equipment, allowing the module to be fastened to the rack frame with screws or bolts. Common uses include computer servers, telecommunications equipment and networking hardware, audiovisual production gear, music production equipment, and scientific equipment.
A full-height rack cabinet
A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany
A professional-grade oscilloscope Tektronix 7603 (1970s) for use in electronics and scientific laboratories. Typically for rack-mounted devices, the width of the front panel exceeds the width of the device itself, which provides the overlap zone (including screw holes) with the left and right rack rails.
Example of 19-inch computer rack with servers
Server is a computer that provides information to other computers called "clients" on computer network. This architecture is called the client–server model. Servers can provide various functionalities, often called "services", such as sharing data or resources among multiple clients or performing computations for a client. A single server can serve multiple clients, and a single client can use multiple servers. A client process may run on the same device or may connect over a network to a server on a different device. Typical servers are database servers, file servers, mail servers, print servers, web servers, game servers, and application servers.
Wikimedia Foundation rackmount servers on racks in a data center
The first WWW server is located at CERN with its original sticker that says: "This machine is a server. DO NOT POWER IT DOWN!!"
A rack-mountable server with the top cover removed to reveal internal components
A server rack seen from the rear