Eurocard is a European standard format for PCB cards, which can be plugged together into a standardized subrack. The subrack consists of a series of slotted card guides on the top and bottom, into which the cards are slid so they stand on end, like books on a shelf. At the "back" of each card is one or more connectors, which plug into mating connectors on a backplane in the rear of the subrack.
The Eurocard height starts at 3U, i.e. 100 millimetres (3.937 in) and increments by 5.25 inches (133.350 mm) increments. This arrangement allows two 3U Eurocards to be supported properly next to a 6U high (233.35 millimetres (9.187 in)) Eurocard. The 33.35 millimetres (1.313 in) space is required for the card guide structure that would be between the two 3U high cards.
Eurocards come in depths that start at 100 millimetres (3.937 in) and then increase in 60 millimetres (2.362 in) increments. The 160 millimetres (6.299 in) depth is the most common today, however standard hardware is available to accommodate depths of 100 millimetres (3.937 in), 160 millimetres (6.299 in), 220 millimetres (8.661 in), 280 millimetres (11.024 in), 340 millimetres (13.386 in), and 400 millimetres (15.748 in).
- Eurocard article on Wikipedia