The inner angle of the same side, "the same side" refers to the same side of the third straight line; "Inside" means between two straight lines cut.

Among the eight angles formed by two straight lines being cut by the third straight line, there are four pairs of congruent angles, two pairs of internal staggered angles and two pairs of internal angles on the same side.

Theorem: 1 Two straight lines are parallel and their internal angles are complementary.

2. The internal angles on the same side are complementary and the two straight lines are parallel.

alternate interior angle

Two straight lines AB and CD are cut into eight angles by a third straight line EF. If both angles are on the inside of two straight lines and on both sides of the third straight line, then such a diagonal is called an inscribed angle. As shown in the figure, ∠3 and ∠6, ∠4 and ∠5 are all internal angles.

Vertical diagonal

Diagonal angle: Two angles with only one common vertex and no common edge obtained by the intersection of two straight lines are called antipodal angles, and the intersection of two straight lines forms two pairs of antipodal angles. The two opposite angles are equal. Diagonal angle is the name of two special angles; Equal vertex angles reflect the size relationship between two angles.