**Definition:** **Angles**** are created at the intersection between two lines. As seen in the following illustration**

The angle in the illustration is called $AB$. We could also call it angle $\sphericalangle ABC$. The important thing is that the middle letter is the one at the intersection of the lines.

**For example, in this case:**

The angle is $\sphericalangle BCD$ or $\sphericalangle DCB$. Both notations are correct for the same angle.

**We usually mark the angle with an arc as follows:**

**The marked angle is** **$∡ABC$****. Sometimes we will denote angles using Greek letters, for example:**

$α$ or $β$

**Before the name of the angle, we should note the angle symbol, like this:**

$∡$

**Together it looks like this:**

$∡CBA$ or $∡α$

Next, we will delve into the size of angles, the different types, and those that are created when a line intersects two parallel lines.