Section 01
Pairs of angles formed when a straight line intersects two straight lines
A straight line that intersects two or more straight lines at two different points is called a transversal. The intersection produces eight angles: four interior and four exterior.
Transversal t cuts lines r and q. Highlighted: ∠1 and ∠5 — corresponding angles.
Interior angles
Located between the two lines: ∠3, ∠4, ∠5, ∠6.
Exterior angles
Located outside the two lines: ∠1, ∠2, ∠7, ∠8.
Corresponding angles
Same side of the transversal, one exterior and one interior, and not adjacent.
Alternating interior angles
Both interior, on opposite sides of the transversal, and not adjacent.
Alternating exterior angles
Both exterior, on opposite sides of the transversal, and not adjacent.
Interior angles on the same side
Both interior and on the same side of the transversal (also called co-interior angles).
How to spot each pair quickly
Look for an F shape
The two angles inside an F (in any rotation) are corresponding angles.
Look for a Z shape
The two angles inside a Z (or N) are alternating interior angles.
Look for a C or U shape
The two angles inside a C are interior angles on the same side of the transversal.
Check adjacency
A pair is never adjacent — two angles sharing a ray at the same point form a linear pair instead.