# Parallel and Perpendicular Lines

### Terms

### Alternate Exterior Angles

The pair of angles on the outside of the two lines cut by the transversal and on alternate sides of the transversal. Alternate exterior angles are congruent if and only if the two lines crossed by the transversal are parallel.### Alternate Interior Angles

The pair of angles in between the two lines cut by the transversal and on alternate sides of the transversal. Alternate interior angles are congruent if and only if the two lines crossed by the transversal are parallel.### Consecutive Exterior Angles

The pair of angles on the outside of the two lines cut by the transversal and on the same side of the transversal. They're also called same-side exterior angles, for obvious reasons. Consecutive exterior angles are supplementary if and only if the two lines crossed by the transversal are parallel.### Consecutive Interior Angles

The pair of angles in between the two lines cut by the transversal and on the same side of the transversal. They're also called same-side interior angles, for obvious reasons. Consecutive interior angles are supplementary if and only if the two lines crossed by the transversal are parallel.### Corresponding Angles

Two angles that are in the same relative place compared to each of the two lines and the transversal that cuts them. Corresponding angles are congruent if and only if the two lines crossed by the transversal are parallel.### Parallel

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?### Parallel Postulate

Euclid's fifth postulate, and it's quite a tricky one compared to the previous four. It states that if two lines are crossed by a third and both angles on the interior and same side add up to less than the sum of two right angles, the two original lines will eventually intersect on that side. What a mouthful, Euclid. Sheesh.### Perpendicular

Two lines that intersect at exactly 90°, forming four right angles.### Polygon

A closed two-dimensional shape that is made of only straight line segments. No curves allowed. Sorry, Beyoncé.### Regular Polygon

A shape whose sides are all equal in length and whose angles are all equal in measure.### Transversal

A line that intersects two other lines, forming a total of eight angles. If the other two lines are parallel (and they usually are), then all these angles are special in some way.Advertisement

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