An angle less than 90 is an acute angle, an angle equal to 90 is a right angle, and an angle greater than 90 and less than180 is an obtuse angle. An angle equal to 180 is called a flat angle. More than 0 and less than 180 are called bad angles, and acute angles, right angles and obtuse angles are all bad angles. More than 180 and less than 360 is called the optimal angle.

Related concepts of angles complementary angle and complementary angle: if the sum of two angles is 90, it is complementary angle, and if the sum of two angles is180, it is complementary angle. The complementary angles of equal angles are equal, and the complementary angles of equal angles are equal.

Inverse vertex angle: When two straight lines intersect, there is only one common vertex, and both sides of the two corners are opposite extension lines. These two angles are called antipodal angles. Two straight lines intersect to form two pairs of vertex angles. The two opposite angles are equal.

Adjacent complementary angles: two angles have a common edge, and their other edge is an opposite extension line. Two angles with this relationship are adjacent complementary angles.

Internal angle: Two straight lines are cut by a third straight line. If both angles are on the inside of two straight lines and on both sides of the third straight line, then such a pair of angles is called an inner angle.

Inner angle on the same side: two angles are on the same side of the dividing line, and between the two dividing lines, a diagonal with this positional relationship is the inner angle on the same side.