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Points, Lines, Angles, and Planes Terms

Get down with the lingo


An adorable little angle. By "little," of course we mean "smaller than 90 degrees in measure."


The conjoined twins of the geometry world. Adjacent angles always share a vertex and a side.


The corner of empty space between two lines, rays, or segments that share a point. Usually, angles are measured in degrees (and most of them have at least a bachelor's degree).

Angle Addition Postulate

The idea that the sum of the measures of two adjacent angles is the measure of the angle that contains those adjacent angles.


Split in two parts of equal size. We can say that an angle is bisected, a segment is bisected, or that the Dakotas are bisected.


Something that splits another thing in half. Like a ray being an angle bisector, a segment bisecting another segment, or a chainsaw bisecting a zombie.


Used to describe points that are all on the same line.

Complementary Angles

Two wrongs don't make a right, but two complementary angles do. They're two angles that add up to 90° exactly.


Exactly equal in measure or identical. #twinsies


Used to describe lines or points that are all on the same plane.

Distance Formula

A formula that calculates distances based on the coordinates we plug into it. It takes the form of this lovely thing: .


The distance between two endpoints. Can be expressed in units of millimeters, inches, feet, miles, or light-years—they're all lengths.


A one-dimensional segment that continues on forever in both directions. Time-consuming to draw, so we use arrows on the ends to symbolize that it never ends.

Line Segment

Sometimes just called a "segment." It's a finite piece of line between two endpoints.


A point that bisects a line segment into two congruent segments. It's the halfway point.


An angle larger than 90° in measure. It has nothing to do with their smarts.


The point (0, 0) on the x-y graph. In 3D space, it's (0, 0, 0). For any additional dimensions, just add more zeros.


Two lines that are on the same plane but never intersect. They're always in sight, but never touch…sort of sad, ain't it?


Two lines that create a 90° angle when they intersect. Well, actually they create four 90° angles, but who's counting?


A "slice" of three-dimensional space. It has length and width, but no depth, like a sheet of paper that stretches out forever in all directions.


The smallest object…ever. It has no mass, no length, and no size. It describes only a location.


A hybrid of a line and a segment. It has one endpoint, but then goes off forever in the other direction. It's like a ray of sunshine that starts at the sun and then continues on forever.

Right Angle

An angle that measures exactly 90°. Naturally, that means any angle that doesn't measure 90° is wrong.

Segment Addition Postulate

The idea that we can add segments together if they're part of the same line. If a Taco Bell is in between your house and your school, then the distance from your house to your school is the same as the distance from your house to the Taco Bell plus the distance from the Taco Bell to the school.

Straight Angle

An angle that measures 180°. The title a straight line gives itself when it wants to seem more impressive.

Supplementary Angles

Two angles that add up to 180°. It doesn't matter if they're adjacent or on different planets.

Undefined Notions

Points, lines, and planes. While we can describe them in great detail, there's a slight problem in defining them in that we can't.

Vertical Angles

Angles that are opposite each other resulting from two intersecting lines. Vertical angles are always congruent.

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