## 1.1 Understanding Points, Lines and Planes

**Point: Location without size**

Line: Straight path without thickness, extends forever

Plane: Flat surface without thickness, extends forever

Segment: Part of line consisting of two points, and all points in between

Endpoint: A point at one end of a segment or the starting point of a ray

Ray: Part of a line that starts at an endpoint and extends forever in one direction

Opposite Ray: 2 rays that have a common endpoint

Postulate 1-1-1 Through any 2 points, there is exactly one line

Postulate 1-1-2 Through any 3 non-colinear points, there is exactly one plane containing them

Postulate 1-1-3 If 2 points lie in a plane, then the line containing those points lies in the plane

Line: Straight path without thickness, extends forever

Plane: Flat surface without thickness, extends forever

Segment: Part of line consisting of two points, and all points in between

Endpoint: A point at one end of a segment or the starting point of a ray

Ray: Part of a line that starts at an endpoint and extends forever in one direction

Opposite Ray: 2 rays that have a common endpoint

Postulate 1-1-1 Through any 2 points, there is exactly one line

Postulate 1-1-2 Through any 3 non-colinear points, there is exactly one plane containing them

Postulate 1-1-3 If 2 points lie in a plane, then the line containing those points lies in the plane

**Postulate 1-1-4 If 2 lines intersect, then they intersect at exactly one point**

Postulate 1-1-5 If 2 planes intersect, they do so at one line

Postulate 1-1-5 If 2 planes intersect, they do so at one line