 The extension of the concept of continuous symmetry from “global” symmetries (such as the Galilean group of spacetime transformations) to “local” symmetries is one of the important developments in the concept of symmetry in physics that took place in the twentieth century. Prompted by GTR, Weyl’s 1918 “unified theory of gravitation and electromagnetism” extended the idea of local symmetries (see Ryckman, 2003, and Martin, 2003), and although this theory is generally deemed to have failed, the theory contains the seeds of later success in the context of quantum theory (see below,  Section 2.5).