"the smooth gold metal of the Pennsylvania keystone"
A keystone shape
Permission Granted by Copyright Owner for Use on Book DrumA keystone shape - Credit: Susannah Worth

A keystone shape is illustrated here. Pennsylvania's association with the motif is long-standing but its origins aren't entirely clear. A keystone is the wedge-shaped stone at the top of an arch that locks the structure in place; Pennsylvania's central position on the Eastern Seaboard of the USA – geographically, economically and in historical importance – may be the reason for the association.