Angle
When two straight lines meet at a common point, they form an angle.
and 
are called arms of the angle BAC. The point 'A' is called the vertex. The angle formed by the two rays AB and AC is denoted by the symbol 
BAC. If there is only one angle at A, then the angle BAC may be denoted by 
In a given plane two lines intersect at only one point. The angles are measured with the help of a protractor. The measure of an angle is called its magnitude.
Acute Angle
An angle whose measure is less than 90o is called an acute angle.
Right angle
It is an angle whose measure is equal to 90o.
Obtuse angle
An angle whose measure is greater than 90o but less than 180o is called an obtuse angle.
Straight angle
An angle with measure equal to 180o is called a straight angle.
Reflex angle
It is an angle with measure greater than 180o but less than 360o.
Angle relations
Adjacent angles
A pair of angles having a common arm and a common vertex.
and 
are adjacent angles.
O is the common vertex.
Complementary angles
They are a pair of angles, the sum of whose measures is equal to 90o. These may be adjacent angles as well.
(These are not adjacent angles)
Supplementary angles
They are a pair of angles, the sum of whose measures is equal to 180o. These may be adjacent angles as well.
[See figure below]
[See figure below]
Linear pair of adjacent supplementary angles
If a pair of angles are adjacent as well as supplementary then they are said to form a linear pair.
are adjacent angles,
and 
[See figure below]
