Capsquare

In artillery, capsquares, or capsquires, are strong plates of iron which come over the trunnions of a cannon, keeping it in its carriage. They are fastened by hinges to the paizeplate, so that it could be lifted up and down. They form a part of an arch in the middle, to receive a third part of the thickness of the trunnions; for two thirds of them are let into the carriage, and the other end is fastened by two iron wedges, which are called the forelocks, and keys.

An old capsquare (both top and bottom pieces)

 This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "Capsquare". Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al.