Alternate interior angles are created when 2 parallel lines are crossed by another line. This line is called a transversal. Alternate interior angles are inside the parallel lines and on opposite sides of the transversal.

This diagram shows 2 pairs of alternate interior angles: 

• Angles and
• Angles and

There are two horizontal parallel lines, and a third diagonal line drawn from the bottom left to the upper right, intersecting both horizontal lines. The diagonal line is labeled transversal. There are four angles created by the diagonal line inside the parallel lines. The upper left angle is labeled a, upper right is b, lower left is c, and lower right is d.

Complementary angles have measures that add up to 90.

For example, a angle and a angle are complementary.

One figure is congruent to another if it can be moved with translations, rotations, and reflections to fit exactly over the other.

In this figure, Triangle A is congruent to Triangles B, C, and D.

• A translation takes Triangle A to Triangle B.
• A rotation takes Triangle B to Triangle C.
• A reflection takes Triangle C to Triangle D.

Corresponding parts are the parts that match up between a figure and its scaled copy. They have the same relative position. Points, segments, angles, or distances can be corresponding.

Point in the first triangle corresponds to point in the second triangle. Segment corresponds to segment .

An image is the result of translations, rotations, and reflections on an object. Every part of the original object moves in the same way to match up with a part of the image.

Triangle has been translated up and to the right to make triangle . Triangle is the image of the original triangle .

A reflection is a transformation that “flips” a figure over a line. Every point on the figure moves to a point directly on the opposite side of the line. The new points are the same distance from the line as they are in the original figure.

This diagram shows a reflection of A over line that makes the mirror image B.

A rigid transformation is a move that does not change any measurements of a figure. Translations, rotations, and reflections are rigid transformations. So is any sequence of these.

A rotation is a transformation that “turns” a figure. Every point on the figure moves around a center by a given angle in a specific direction.

This diagram shows Triangle A rotated around center by 55 degrees clockwise to get Triangle B.

A sequence of transformations is a set of translations, rotations, reflections, and dilations on a figure. The transformations are performed in a given order.

This diagram shows a sequence of transformations to move Figure A to Figure C. 

First, A is translated to the right to make B. Next, B is reflected across line to make C.

Supplementary angles have measures that add up to 180.

For example, a angle and a  angle are supplementary.

A tessellation is a repeating pattern of 1 or more shapes. The sides of the shapes fit together with no gaps or overlaps. The pattern goes on forever in all directions.

This diagram shows part of a tessellation.

A transformation is a translation, rotation, reflection, or dilation, or a combination of these.

A translation is a transformation that “slides” a figure along a straight line. Every point on the figure moves a given distance in a given direction.

This diagram shows a translation of Figure A to Figure B using the direction and distance given by the arrow.

A transversal is a line that crosses parallel lines.

This diagram shows a transversal line intersecting parallel lines and .

Vertical angles are opposite angles that share the same vertex. They are formed when two lines cross each other. Their angle measures are equal.

Angles and are vertical angles. If angle measures , then angle must also measure .

Angles and are another pair of vertical angles.