Two lines are perpendicular if they intersect to form a right angle. A line perpendicular to a plane is perpendicular to every line it intersects in that plane. Definitions of geometric terms can be interpreted both ways, such as defining perpendicular lines as those that form right angles. Biconditional statements relate two statements that are logically equivalent, using "if and only if" or "iff", and are true when the statement and its converse are both true.

1. Section 2.2
Also includes perpendicular and parallel lines

2. Perpendicular and Parallel
 Perpendicular lines intersect to form a right angle
 A line that is perpendicular to a plane is perpendicular
to every line in that plane that it intersects
 Definitions, such as these, are meant to be interpreted
in both directions.
 For example, the first definition could be written
“Two lines that form right angles are perpendicular”

3. Biconditional Statements
 Like definitions, biconditional statements can be
interpreted in both directions
 They are written in the form:
<1st statement> if and only if <2nd statement>
 “If and only if” is often abbreviated “iff”
 One can make a biconditional statement if the
conditional statement and its converse are both true
(or both false, but why would you do that)

4. Reference
 McDougal Littel: Geometry, Section 2.2