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Know the proper name for pair door types.

The image at right shows illustrations of different pair door types with names beneath.

The top illustrations are named Common Pairs because each door in the pair could be used individually as is. Specifically, each arched panel door, seen on the right, could stand alone as a single door.

The bottom illustrations show Common Arch Pairs (CAP), so named because the panels share a common arch. The illustration in the bottom right is an Arched Top Common Arch Pair (AT-CAP). In the case of both bottom illustrations, the individual door slabs in each pair would not be used alone, because the common arch would appear incomplete.