The term “U” stands for “rack unit” and is used to describe the height of a module or the size of a case or cabinet. Rack units are a standardized measurement system used in various equipment racks, including audio, networking, and server racks.
In modular synthesis, the most common rack unit size is 3U, which refers to the Eurorack format. One rack unit (1U) is equal to 1.75 inches (44.45 mm) in height. Therefore, a 3U Eurorack module or case has a height of 5.25 inches (133.35 mm). This standardization allows for consistent sizing and compatibility among different manufacturers’ modules, cases, and cabinets.
There are also other modular synthesizer formats that use different rack unit sizes, such as 4U (Buchla or Serge) and 5U (Moog Unit or MU). Each format has its own set of conventions regarding module height, width, power, and connection types.