The butterfly table is without doubt the coolest of expanding table designs. Its hinged leaf lies under the tabletop and swings up and open, coming to rest fully expanded on the aprons. The leaf itself is two pieces joined by barrel hinges, connected to the table by a pair of pivot pins. The top can be rectangular, round, oval, or just about any shape. The two halves of the top are attached to T-shaped slides that ride in notches cut into the end aprons, and a pair of cross-stretchers that run between the aprons. A small shelf supports the leaf when it is stored under the table.

From Fine Woodworking #220 with p[roject plans