Eurocard is a European standard format for PCBs, 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 format does not define specific connectors to be used nor the signals that are assigned to connector contacts.
Eurocards come in depths that start at 100mm and then increase in 60mm increments. The 160 millimetres depth is the most common today, followed by 220mm. However standard hardware is available to accommodate depths from 100mm to 400mm.
A single size eurocard is 100mm x 160mm (3U) and double size eurocard is 233.35mm x 160mm (6U). The extra 33.35mm allows two 3U Eurocards to be supported properly next to one 6U high, with the width taken up by a card guide between the two 3U cards.
- Eurocard article on Wikipedia