Frac is a modular synthesizer format. Frac is sometimes referred to as "Frac Rack"
Frac is considered a 3U format. The height of most modern modular synthesizer modules is measured by the number of vertical units it consumes in a standard 19" equipment rack. Modules conforming to the 3U format, for example, will take up three vertical units in a 19" equipment rack.
Maunfacturers of Frac format synthesizer modules include Paia and Blacet.