Grade 9 Geometry - Basic Terms and Angle Relationships

Vocabulary based on Grade 9 Geometry notes covering undefined terms, subsets of lines, angle relationships, and properties of parallel lines cut by a transversal.

Point

A location in space typically described as having no size or dimension, named using capital letters.

Line

A set of points that has one dimension, extends infinitely in both directions, and can be named using two points on the line or a lowercase letter.

Plane

A flat, two-dimensional surface that extends infinitely in all directions, named using three noncollinear points or a script capital letter.

Line Segment

A part of a line that has two endpoints and a fixed length.

Ray

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

Angle

A geometric figure formed by two rays (sides) that share a common endpoint called a vertex.

Geometric Construction

The process of drawing geometric figures accurately using only specific tools such as a straightedge and a compass.

Straightedge

A tool used only for drawing straight lines or connecting points.

Compass

A tool used to draw circles, arcs, and equal distances accurately.

Parallel Lines

Straight equidistant lines that lie on the same plane and do not intersect or meet regardless of how far they are extended.

Perpendicular Lines

Two lines that intersect to form a perfect $90$-degree angle (right angle).

Vertical Angles

Nonadjacent angles formed by two intersecting lines that are congruent.

Linear Pair

Two adjacent angles whose noncommon sides are opposite rays; they are supplementary.

Supplementary Angles

Two angles whose degree measures sum to $180^\circ$.

Complementary Angles

Two angles whose degree measures sum to $90^\circ$.

Congruent Angles

Two angles that have the same degree measure.

Transversal

A line, line segment, or ray that intersects two or more lines at different points.

Alternate Exterior Angles

Congruent angles that lie on opposite sides of the transversal and lie outside the two lines.

Same-side Interior Angles

Supplementary angles that lie on the same side of the transversal and inside the parallel lines.

Same-side Exterior Angles

Supplementary angles that lie on the same side of the transversal and outside the parallel lines.

Corresponding Angles

Congruent angles that lie on the same side of the transversal and on the same side of the two lines.

Alternate Interior Angles

Congruent angles on opposite sides of the transversal and inside the two parallel lines.