DEC card form factor
The original small standard length FLIP CHIPs (used in the PDP-8 and KA10 PDP-10 processor) had a total board size of 4.93 inches (from the far end of the edge connector contacts, to the handle end of the board) by 2.437 inches (from one side to the other). From this basic size, a large array of other sizes was built up.
A board of the same width, but approximately twice as long (8.43 inches) was referred to an an extended length board; many later M-series FLIP CHIPs used this format. A board of the short length, but twice as wide (5.187 inches) was referred to as a double height board; boards of this size are not common, but do exist.
(The 'height' does not refer to the distance from the edge with the handles, to the edge with the contacts, but rather to the distance from one side edge to the other; this is because these board were usually mounted vertically, with the side edges on the top and bottom.)
Extended length boards came in multiples of two, four (10.457 inches) and six times the width of the small original board; these are usually called dual, quad and hex height format boards.
Finally, as production processes improved, and leading signals from board to board became undesirable, even longer hex boards were introduced; these first appeared in the VAX-11/780, and were called super hex boards within DEC.