The PCBs used in PDP-11 series machines and interfaces come in four basic sizes :-

• Single height (2.4 in., 6.1 cm)
• Dual height (5.1 in., 5.2 cm)
• Quad height (10.4 in., 26.5 cm)
• Hex height (15.7 in., 39.8 cm)

The height refers to the number of connector fingers on the board. Each finger has 18 connections on each side. The standard module length is 8.8 inches (22.6 cm), although shorter boards were sometimes used in controllers (5.4 in., 13.7 cm). Hex cards had a full length brace with lever fingers on the ends as did quad Q-bus cards, while the other had colour coded plastic handles.