A modular synthesizer format which falls into the "small format" category. Not to be confused with various standards for industrial and lab instrumentation which use the same name (or with the line of low-cost mixers marketed by Behringer under this name), this Eurorack format calls for modules of 128.5mm (slightly over 5") height. Horizontal width is measured in "horizontal pitch", where 1 HP = 5mm (very slightly less than 0.2"); most manufacturers use a horizontal unit definition of either 1U = 4HP, or 1U = 6HP. 3.5mm phone jacks are used for interconnection.
Unlike most formats used by current modular manufacturers, the Eurorack format calls for ±12V power (most other current formats use ±15V), in addition to +5V required by some modules. Unfortunately, the power connections are not well standardized; the format uses ribbon cables for power and a two-row ribbon cable connector containing either 10, 12, or 16 pins. The 16-pin connector uses some of the extra pins to distribute control voltage and gate signals from a keyboard to the modules via a common case connection.