A trammel is a device consisting of two adjustable points attached to a beam or rod. It is used to draw or lay out an arc or a circular shape with a fixed radius. In dome construction, the trammel is set to the desired radius of the dome, and as it pivots around a fixed point, it allows the bricklayer to mark the circular pattern on the ground or the base of the dome. This marking serves as a guide for the bricklaying process, helping the mason place the bricks or stones accurately to form the curved shape of the dome. The trammel ensures that the dome’s curvature is consistent and matches the intended design.