The aim of 16-19 Mathematics has been to produce a course which, while challenging, is accessible and enjoyable to all students. The course develops ability and confidence in mathematics and its applications, together with an appreciation of how mathematical ideas help in the understanding of the world and society in which we live. The 16-19 Mathematics syllabus approved by SEAC for first examination in 1996 includes a unit on Locus. This book supports the new unit. It introduces the general process of considering loci, and then moves on to consider mathematical definitions and expressions of common loci. Various definitions of conics are covered, including algebraic treatment. Finally, there is a list of possible topics for investigation. There is no separate unit guide for this unit; it is included in the student text.