Every shape in geometry is built from three things: points, lines and planes. They're the alphabet — once you know them, every theorem is a sentence written with them.
The three primitives
- Point — a position with no size at all. Drawn as a tiny dot, named with a capital letter (A, B, P).
- Line — straight, infinite in both directions. Two points are enough to fix one — line AB.
- Plane — a flat surface that goes on forever. Three non-collinear points lock it down.
Useful relatives
- Ray — half a line. Starts at a point, goes forever the other way.
- Line segment — a piece of a line, with two endpoints.
- Collinear — points sitting on the same line.
- Coplanar — points sitting in the same plane.
Two distinct points always determine exactly one line. Two distinct lines either meet at one point, are parallel (never meet), or — in 3D — skew (don't meet and aren't parallel).