2U, 2 Unit Height

A rack unit is a unit of measure used to describe the height of a server, network switch or other similar device mounted in a 19-inch rack. One rack unit is 44.45 mm (1.75 in) high. more...

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One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a piece of rack mounted equipment is usually described as a number in "U". One rack unit is also sometimes referred to as "1RU".

Professional audio and video gear often comes with rack mount options and use the same measurement specifications.

Half-rack units typically describe units that fit in a certain number of RU, but occupy only half the width of the rack (9.5 inches / 241 mm). For example, a "4U half-rack" DVCAM deck would occupy 4U space (4 × 1.75 inches) height × 9.5 inches width. In theory, two half-rack decks could occupy the 4U space.

Coincidentally, a rack unit is equal to a vershok, which is an obsolete Russian length unit.

Read more at Wikipedia.org