Kinds of angle
Angles are classified according to their degree measure.
(i)    Acute Angle : An angle smaller than a right angle is called an acute angle.

(ii)    Right Angle :  An angle which measures 90º  is called a right angle. In Fig. ABC is a right angle.

(iii)    Obtuse Angle : If an angle is larger than a right angle, but less than a straight angle, it is called an obtuse angle.

(iv)     Straight Angle : An angle which measures 180° is called a straight angle in figure. ABC is a straight angle.

(v)     Reflex Angle : An angle which measures greater than 180° and less than 360° is called a reflex angle. AOC is a reflex angle.

(vi)     Complete Angle : An angle which measures 360° is called a complete angle. AOB is complete angle.

Perpendicular lines
When two lines intersect and the angle between them is a right angle, then the lines are said to be perpendicular. If a line AB is perpendicular to CD, we write AB CD.

Perpendicular bisector
A line perpendicular to the given line segment as well as bisects it,  is called perpendicular bisector. Line LM is perpendicular bisector of line segment AB.