Keystone
in masonry, the central voussoir at the vertex of an arch. The row or course of said stones along the crown of an arch is the key-course. In chromo-lithography, the stone on which a general outline of the picture is drawn or photo-lithographed. Its object is to furnish a guide for the production of the work upon the several color stones.
Transfers from the keystones in common printing inks, more properly called "offsets" to distinguish them from transfers from which impressions can be printed, are made upon as many stones as there are colors required.