
Concurrent Lines
Three or more lines are concurrent, if and only if, they are coplanar, and intersect in a single point.

Parallel Lines
Two lines are parallel, if and only if they are coplanar, and do not intersect.

Skew Lines
Two lines are skew, if and only if they are noncoplanar, and do not intersect.

Perpendicular Lines
Two lines are perpendicular, if and only if, they intersect to form a right angle.

Opposite Rays
Two rays, BA and BC are considered opposite rays, if and only if, B is between A and C.

Betweenness of Rays
For three coplanar rays, AB, AC, and AD, with coordinates, b, c, and d respectively, AC is between AB, if and only if, either b < c < d, or d < c < b.

HalfLine
A set of points is a halfline if and only if, it is one of the two sets of points on either side of a specific point, called the separation point, on a line.

Ray
The set of points, designated AB is a ray, if and only if, it is the union of the separation point A, called the endpoint, and the set of points contained in the halfline, which includes the point B.

Opposite Rays
Two rays, BA and BC are considered opposite rays, if and only if, B is between A and C.

Betweenness of Rays
For three coplanar rays, AB, AC, and AD, with coordinates b, c, and d respectively, AC is between AB, if and only if, either b < c < d, or d < c < b.

Line Segment
A set of points is a line segment, if and only if, it is the union of two points on a line, and the set of all the points between them.

Length of a Line Segment
A given real number is the length of a line segment, if and only if, that real number represents the distance between the endpoints of the line segment.

Equal Line Segments
Two line segments are considered equal, if and only if, the real numbers representing their lengths are equal.

Midpoint of a Line Segment
A point on a line segment is the midpoints of the segment, if and only if, the point is between the endpoints, and divides the given segment into two congruent segments.

Bisector of a Line Segment
Any point, line segment, ray, line, or plane, is called a bisector of a line segment, if and only if, it intersects the line segment in the midpoint of the line segment.

