A rack is a frame or enclosure in which servers, networking equipment, and associated equipment is mounted.
The industry standard for racks is the 19-inch rack, so called because it mounts equipment 19 inches wide.
Racks are measured vertically in rack units (U) of 1.75 inches. The most common full-size racks used in data centers are 42U tall, though smaller racks are available for specialized applications.
For questions about the Ruby framework, use the tag ruby-rack.