1U = 1 3/4 in = 44.45 mm. The unit of height measurement of a 19" equipment rack. These racks are standard for a very large range of electronic devices; my acquaintance with them comes from electronic music, but others may know them from server farms. If you've never seen one, look at a good quality stereo component and imagine the faceplate extended on either side by additional metal with mounting holes for screws (these are called "rack ears"); that's roughly a 2U rack box.

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