This document defines key concepts in geometry including perpendicular lines, perpendicular planes, and biconditional statements. It explains that a biconditional statement can be rewritten as a conditional statement and its converse. A true biconditional statement is one where both the conditional and converse are true. Examples are provided of writing definitions and postulates as biconditional statements. Students are asked to practice identifying whether statements are biconditional and if they are true.