This is one of a series of geometry modules developed for use by secondary students in a laboratory setting. The authors of this sequence of modules have chosen not to cover the initial basic postulates and theorems since they can be found in nearly every geometry text. Instead, a list of necessary postulates and their consequences is included. It is recommended that the individual instructor using this module sequence use a geometry text of his/her choosing to study the recommended postulates and theorems. After the postulates and theorems have been studied, attack the next module "Triangle Congruence." This module, therefore, gives the student an adequate experience in traditional Euclidean, deductive proof, and teachers should emphasize the nature and place of this power system in the development of geometry. (Author/MK)