Straight Angle - Meaning, Properties, Examples - Cuemath

Straight Angle

A straight angle measures 180° and looks like a straight line, therefore, it is a mathematical way of expressing a straight line. It is an angle whose arms lie in opposite directions from the vertex and they join together to form 180°. Let us learn more about the straight angle in this article.

1.	Straight Angle Definition
2.	Properties of a Straight Angle
3.	Straight Angle Degree
4.	Drawing a Straight Angle Using a Protractor
5.	FAQs on Straight Angles

Straight Angle Definition

Whenever two rays join together they form an angle and the angle subtended by two rays in opposite directions is called a straight angle. In geometry, a straight angle is an angle whose vertex point has a value of 180 degrees.

In other words, 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.

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.

Straight Angle Degree

The degree or the measure 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. Observe the figure given below to understand this fact.

Drawing a Straight Angle Using a Protractor

A straight angle can be constructed easily using a protractor. Follow the steps given below to draw a straight angle 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 line AB such that the baseline of the protractor is over the line AB. Ensure that B is pointing to 0°.
• Step 3: Starting from 0° in the inner scale and keeping the protractor fixed on its place, move your pencil slowly until you reach 180°. Mark this point as C and also mark an arrowhead here.
• Step 4: Join point C with line AB. Thus, a straight angle CAB of 180º is formed.

The following figure shows how a straight angle is constructed using these steps.

Straight Angle Pair

A pair of angles that 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 pairs 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 a common arm and a common vertex. In the following figure, QS is the common arm and Q is the common vertex.

Straight Angles in Real Life

We come across straight angles often in our day-to-day life. Some examples of a straight angle are given below.

• The clock showing 6 o'clock forms a straight angle.
• A rectangular tabletop represents a straight angle.
• Your line of sight is the perfect example of a straight angle.

All the angles given below are straight angles. They differ only in orientation. Following these, we can find many straight angles around us.

Topics Related to Straight Angles

• Acute Angle
• Central Angle
• Straight Line
• 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.
• In radians, the straight angle is denoted by π.
• 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).