It is perhaps a rather unfortunate and possibly misleading fact that the standard list of theorems in geometry contains two which are specially labelled “Theorems on Loci” and form a section to themselves, viz. Section 3 in the I.A.A.M. schedule. It is liable to suggest that the notion of a locus and the properties of loci form an isolated subject like a chapter, say, on quadratic equations, which do not concern us until we reach it, and when this moment arrives we have merely to do our two theorems and a few examples and then may regard the subject as disposed of and consider no further reference is needed except possibly in a revision of proofs of theorems before taking School Certificate.