Eurorack

Euro is a modular synthesizer format, (sometimes called "Eurorack"). The height of Euro format synthesizer modules is 5.25", which is equal to three vertical units in a standard 19" equipment rack. (This standard height is called "3U".) Euro modules typically use 3.5mm connectors and plugs for patching. Power requirements for Euro modules are usually +/-12VDC and 5VDC.

Euro module widths are typically measured in "HP" (or "TE"); 1 TE/HP is equivalent to 5.08mm, or 1/5 inch. So a 24 TE/HE distance is 24/5, or 4.8". In millimeters, it becomes 24 x 5.08=122mm.

Refer to the List of Modular Synth Equipment Manufacturers page for a list of Euro module manufacturers.

Module Depth
Following is a list of Euro module depths. This information is important when purchasing or planning construction of a case or rack for your modules. (courtesy of felix, from this thread)

Livewire  Frequensteiner - 1.50" (38.1mm) Dual Cyclotron - 2.25" (57.15mm)

Plan B  Model 28 - 2.50" (63.5mm) Model 15 - 1.75" (44.45mm) (MKII, MKI is much deeper) Model 14 - 3.75" (95.25mm)

The Harvestman  Tyme Sefari - 2.00" (50.8mm) Malgorithm - 1.50" (38.1mm)

Doepfer