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Plane. Straight. Beam

A plane is an ideal “flat surface with no edge”; on paper we draw only a piece of a plane (a sheet, a board), and we place lines and segments on it. A line extends forever in both directions; in a drawing you see only a segment of a line with arrows suggesting continuation. A ray is “half a line”: it has a starting point and goes infinitely in one direction; the drawing shows an arrow that way. A segment is part of a line between two points; both endpoints belong to the segment. Points are named with capital letters: segment ABAB, ray OMOM (vertex OO).
A drawing lies on a piece of a plane
A line is infinite in both directions
A ray — a start and one direction
A segment — two endpoints
Sketches: line, ray, segment
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