The Arch can be frequently found in Freemasonry in different forms and contexts. With a keystone you can find it in the Royal Arch, the Royal Arch degrees of the Ancient and Accepted Scottish Rite, York and other rites and in the Mark degree. Without a capstone it appears in other degrees.
Several meanings can be given to the arch. A connection, a door, sometimes the significance lays in a detail (such as the capstone).
Duncan’s Masonic Ritual and Monitor (1866) by Malcolm C. Duncan