Straight Angle
An angle in which two rays lie in the exact opposite direction is called a straight angle. The angle of a straight angle is usually 180°. Whenever two rays join together they form an angle. So, the angle subtended by two rays in opposite directions is called a straight angle. A straight angle is also called a 'Supplementary angle' or a 'Flat angle'.
Definition of Straight Angle
A straight angle is basically a mathematical way of saying a straight line. When two rays join an angle is formed. The common point where these two rays meet is called the vertex and the rays are called arms of the angle. In geometry, a straight angle is an angle, whose vertex point has a value of 180 degrees.
When the arms of the angle lie in the opposite direction, they form a straight angle. The arms make a straight line through the vertex. An angle of measure 180º is formed where ever the two rays(arms) are joined to each other at endpoints.
Straight Angle Degree
The degree of a straight angle is always 180º. In the conventional angle measuring system, we consider the right angle as 90º and the angle around a point to be 360º. The straight angle is made up of two right angles.
Here are some interesting facts related to the straight angles.
 In radians, the straight angle is denoted by π.
 A 180 degree is also considered as supplementary.
 Your line of sight is the perfect example of a straight angle.
 We also have the famous rule of shooting on this name: 180º rule
How Do We Call a Straight Angle?
A straight angle is also called a "flat angle".Basically, it forms a straight line, whose sides lie in opposite directions from the vertex.
Straight Angles In Real Life
We come across the straight angle often in our daytoday life. Some examples of a straight angle are given in the list below:
 A protractor,
 The clock showing 6 o'clock,
 A flat surface.
All the angles given below are straight angles (they differ only in orientation)
Properties of a Straight Angle
The important properties of straight angles are as follows. A straight angle is formed by rotating one ray by 180° with respect to another ray.
 A straight angle reverses the direction of a point.
 A straight angle is exactly half of a revolution.
 A straight angle can also be formed by joining two right angles.
 In a straight angle, the angle rays face the opposite directions.
Straight Angle Pair
A pair of angles that together form a straight line is called a straight angle pair. The sum of two angles that are in a straight angle pair is always equal to 180°. They are also known as linear pair of angles. The image shown below shows two angles a = 135° and b = 45°, which together form 180°. The straight angle pairs have the following properties in common. They have,
 Common arm
 Common vertex.
Drawing a Straight Angle Using a Protractor
Follow the steps below to draw a straight angle by using a protractor:
 Step 1: Draw a straight line and name it as AB and mark an arrowhead on B.
 Step 2: Place the protractor on the line AB so that the baseline of the protractor is over the line AB. Ensure that B is pointing to 0°.
 Step 3: Starting from 0° rotate slowly until 180° is reached. Mark this point as C and also mark an arrowhead here.
 Step 4: Join point C with line AB. Thus, angle CAB is of 180º.
The figure below shows how a straight angle is constructed using these steps.
Topics Related to Straight Angles
Important Notes:
 A straight angle is formed by rotating one ray by 180° with respect to another ray.
 A straight angle is exactly half of a revolution.
 A straight angle reverses the direction of a point.
 A straight angle is also called a ‘flat angle’.
 A Straight angle pair is a set of two adjacent angles on a straight line that add up to 180°
 If ∠A + ∠B = 180°, then ∠A and ∠B form a straight angle pair(linear pair).
Important Topics
Solved Examples

Example 1: Identify the straight angle pairs in the following figure.
Solution:Here, ∠AOC,∠BOD are straight angles. ∠AOD and ∠COD are having common arm OD,common vertex 'O'. They are on same straight line \(\overleftrightarrow{AC}\). So,∠AOD and ∠COD form a straight angle pair.
∠1+∠4=180°
Simillarly,
∠2,∠3
∠3,∠4
∠1,∠2 also form straight angle.
So, in total, there are 4 straight angle pairs. 
Example 2: Does the following figure represent a straight angle pair?
Solution:
∠1,∠2 have common side and common vertex. As we can observe from the figure, ∠1 and ∠2 are both acute angles. So, ∠1+ ∠2 is not 180°. The noncommon arms do not form a straight angle.
Therefore, ∠1 and ∠2 are not a straight line pair. 
Example 3: If the hour hand of a clock is at 6 and the minute hand is at 12. What is the angle thus formed?
Solution:
The angle between the hands at 6'o clock is 180º. Therefore, the clock shows a straight angle. 
Example 4: Tia states that "If two congruent angles form a linear pair or straight angle pair, the angles are right angles." Check whether she is correct?
Solution:If two angles form a straight angle pair (linear pair) then they both add up to 180º, which means ∠A + ∠B = 180º
Here the angles are congruent,
∠A = ∠ B
∠ A + ∠ B = 180°
2∠A = 180
∠A = 90°
∠A = ∠ B = 90°
So, Tia is correct.
Practice Questions on Straight Angle
FAQs on Straight Angles
What Does a Straight Angle Look Like?
A straight angle looks like a straight line whose angle is 180°.
What is a Straight Angle?
When two rays or arms of an angle are in opposite directions, they make a straight line. The angle subtended by these two rays will be 180°. This is called a straight angle.
What is the Definition of Straight in Geometry?
In geometry, straight means two arms or rays are in opposite directions.How Many Degrees Does a Straight Angle Measure?
A straight angle measures 180°.
What is Half the Measure of a Straight Angle?
90°^{ }is half the measure of a straight angle. It is also called as a right angle.
Can We Say that a Straight Line is a Straight Angle?
Yes, a straight line with two endpoints makes an angle of 180°. So we can consider a straight line as a straight angle. Any point on a line measures 180º.