Lines and Angles — SAT Math Explained
Lines are straight, infinite paths extending in both directions. Angles are formed where two rays meet at a common endpoint (vertex), measured in degrees.
The Core Idea
Angle relationships between intersecting lines and parallel lines cut by a transversal follow strict patterns. Recognizing these relationships lets you find unknown angles without measuring.
Key Vocabulary
Half of a line — starts at a point and extends infinitely in one direction
Formed by two rays sharing an endpoint
The endpoint where the two rays of an angle meet
A line that crosses two or more other lines
Angle Types
Less than 90°
Exactly 90°
Between 90° and 180°
Exactly 180° (a straight line)
Greater than 180°
Angle Relationships
Two angles summing to 90°
Two angles summing to 180°
Opposite angles formed by two intersecting lines — always equal
Share a vertex and a side but don't overlap
Same position at each intersection when a transversal crosses parallel lines — equal
Between the parallel lines, on opposite sides of the transversal — equal
Between the parallel lines, on the same side — supplementary (sum to 180°)
Common Errors to Avoid
Confusing complementary (90°) and supplementary (180°)
Assuming alternate interior angles are supplementary rather than equal
Not recognizing when lines are parallel before applying parallel line angle rules
Practice: Lines and Angles
5 SAT-style questions. Select your answer and get an instant explanation.
Two parallel lines are cut by a transversal. One same-side interior angle is 112°. What is the other?
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