The two angles at which two straight lines intersect with the third line are called inner angles on the same side, which are located on the same side of the cutting line and within the cutting line. 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. These two lines are parallel and complementary.

Adjacent complementary angles (or adjacent angles on a straight line [1]), two angles have a common edge, and their other side is an opposite extension line. Two angles with this relationship are called adjacent complementary angles. (that is, the definition of adjacent complementary angles)

If the sum of the two angles is 90, then we say that the two angles are complementary, or that one angle is complementary to the other. For example:

∠A = 30, ∠B = 60, ∠A+∠B = 90, then ∠ A and ∠ B are complementary angles ".

∠A = 15, ∠B = 75, ∠A+∠B = 90. It is said that ∠ A and ∠ B are complementary angles.