Lines, Segments & Rays Cheat Sheet

Key Facts

• A line through points $A$ and $B$ is written $\overleftrightarrow{AB}$ and extends forever in both directions.
• A line segment with endpoints $A$ and $B$ is written $\overline{AB}$ and has a measurable length $AB$.
• A ray that starts at $A$ and passes through $B$ is written $\overrightarrow{AB}$, and the endpoint must be named first.
• If point $B$ is between $A$ and $C$, then the Segment Addition Postulate says $AB + BC = AC$.
• Parallel lines never meet and are written $\ell \parallel m$.
• Perpendicular lines meet to form right angles and are written $\ell \perp m$.
• Intersecting lines cross at exactly one point unless they are the same line.
• A right angle measures $90^\circ$, and perpendicular lines form four $90^\circ$ angles.

Vocabulary

Line

A straight path of points that extends forever in two opposite directions.

Line Segment

A part of a line with two endpoints and a fixed length.

Ray

A part of a line that starts at one endpoint and extends forever in one direction.

Endpoint

A point at the beginning or end of a segment, or at the starting point of a ray.

Parallel Lines

Lines in the same plane that never intersect, no matter how far they extend.

Perpendicular Lines

Lines that intersect to form right angles measuring $90^\circ$.