## What is an angle

An angle is the **surface existing between two straight lines that start from the same point**. The lines are known as **sides**and to the point, **vertex**.

Take, for example, the hands of a clock. Both the hour and minute hands are joined at the same point (the center of the watch). The other end of the needles separates as time passes, creating angles of different openings.

The angles **are measured in degrees**, which is the separation distance between both lines. The more straight lines open, the greater the degree and surface area of the angle.

To measure the degrees of an angle and, therefore, its opening, a semicircular (or even circular) instrument called *conveyor*. Degrees are represented with a number followed by the º sign: 25º (twenty-five degrees).

## Types of angles

There are different types of angles that are classified under different criteria: their opening, their position and the sum (in addition to their position with respect to geometric figures).

### According to its opening in degrees

This criterion classifies the angles according to the degrees of their opening:

Angle **null**: has 0º as there is no opening between the lines. Angle **sharp**: its opening is less than 90º.Angle **straight**: measures exactly 90º.Angle **obtuse**: this type of angle has an opening of more than 90º and less than 180º.Angle **flat**: has an opening of 180º.Angle **oblique **(reflex or concave): measures more than 180º and less than 360º.Angle **perigonal**: It is the largest of all, as it measures 360º.

### According to your position

This criterion is based on the position of one angle with respect to another:

The **consecutive angles** are those that share the vertex and one of the two sides. **adjacent angles** They share the vertex and one side to add up to 180º. Therefore, if the consecutive ones add up to 180º, they are called adjacent. **opposite angles** They only share the vertex, so their sides do not match.

### Sum of angles

Another classification is made through the result of the sum of two angles:

Angles **complementary**: add up to 90º. That is, they form a right angle. Angles **supplementary**: add up to 180º, a straight angle (result of two adjacent angles)Angles **conjugates**: add up to 360º.

### According to their position in geometric shapes

This criterion is based on the position of an angle with respect to different geometric shapes.

Angles in a circle:

Angle **central**: the vertex of the angle is located at the center of the circle. Angle **signed up**: the vertex touches one of the points of the circle. The same happens with the ends of the sides (they are secants). Angle **semi-enrolled**: the vertex touches one of the points of the circle. One of its sides is secant and the other is tangent.

Angles in polygons:

Angle **inside**: are those found inside a polygon. Angle **abroad**: They are those that are on the outside of a polygon.

See also: