3U

3U is the term used for modules or equipment that is three standard rack units high (133.35mm or 5.25"). 3U modules typically use leads with mono 3.5 mm TRS connectors for patching.

Modules 3U high and a multiple of 2HP wide have been adopted by many manufacturers of synthesiser modules and is known more generally as the Eurorack standard. This is in contrast to the older combination of 5U height and 1/4" leads pioneered by Moog. It is relatively easy to combine Eurorack modules from different companies.