BASIC CONCEPTS OF GEOMETRY
Two or more straight lines are parallel when they are coplanar (contained in the same plane) and
do not intersect; that is, when they are an equal distance apart at every point.
The following facts are used frequently in plane geometry. These facts will help you solve
problems in this section.
The shortest distance between two points is the length of the straight line segment
A straight line segment can be extended indefinitely in both directions.
Only one straight line segment can be drawn between two points.
A geometric figure can be moved in the plane without any effect on its size or
Two straight lines in the same plane are either parallel or they intersect.
Two lines parallel to a third line are parallel to each other.
An angle is the union of two nonparallel rays originating from the same point; this point is
known as the vertex. The rays are known as sides of the angle, as shown in Figure 1.
Figure 1 Angle
If ray AB is on top of ray BC, then the angle ABC is a zero angle. One complete revolution of
a ray gives an angle of 360°.