Learn and know what is mean by **co interior angles** in maths. This is we will study in middle class mathematics i.e. in 6th or 7th or 8th classes.

In geometry, we will study about transversal and also we will study about the different angles which are made by transversal with the two lines. For example, interior angles, **co interior angles**, co exterior angles, exterior angles, alternate interior angles, alternate exterior angles, vertically opposite angles, corresponding angles. In these angles, now we will learn about **co-interior angles**.

**What is mean by an angle?**

As we are discussing about **co interior angles**, so it is better to know first what is angle then we will see co interior angles. Generally we will define an angle as “union of two rays”. In some other way we can also define angle as “two rays with a common initial point is called as an angle”. In these two ways we can define an angle.

**What are co interior angles?**

We know that transversal makes totally 8 angles with the two lines. In those 8 angles, some angles we call it as cointerior angles. First we will learn the definition of **co-interior angles**. The co interior angles are the angles which are lying between the two lines and any one side of transversal i.e. either right hand side or left hand side of transversal are called **co interior angles**. See the below diagram which is showing what are **co-interior angles**.

NOTE:

Remember this important point i.e. If the sum of co-interior angles is supplementary i.e. { 180 }^{ 0 } then the two lines will be parallel lines. This is one of the conditions for checking parallel lines in transversal concept and it is useful in solving problems based on transversal.

Hope that what are cointerior angles is clear.

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