Key Points

A point is a precise location. A line segment is the shortest path between two points A and B, denoted $\overline{AB}$. A line extends indefinitely in both directions, denoted $\overleftrightarrow{AB}$. A ray starts at one point and extends endlessly in one direction, denoted $\overrightarrow{AB}$.

An angle is formed by two rays sharing a common endpoint, called the vertex. The two rays are the arms of the angle. It is named using three points, for example $\angle ABC$, where B is the vertex.

The standard unit for measuring angles is the degree ($^\circ$). A full circle or one complete rotation is divided into $360$ degrees ($360^\circ$).

A protractor is a tool used to measure and draw angles. To measure an angle, place the center of the protractor on the vertex and align the $0^\circ$ line with one arm of the angle.

A right angle measures exactly $90^\circ$. It represents a quarter turn and looks like the corner of a square. Two lines that meet at a right angle are called perpendicular lines.

A straight angle measures exactly $180^\circ$. It looks like a straight line and is equivalent to two right angles. It represents a half turn.

An acute angle is an angle that measures more than $0^\circ$ but less than $90^\circ$. It is smaller than a right angle.

An obtuse angle is an angle that measures more than $90^\circ$ but less than $180^\circ$. It is larger than a right angle but smaller than a straight angle.

A reflex angle is an angle that measures more than $180^\circ$ but less than $360^\circ$. It is the larger angle formed by two rays at a vertex.

A complete angle represents one full rotation and measures exactly $360^\circ$. The starting and ending arms of the angle coincide.

The angle between two consecutive numbers on a clock is $30^\circ$, which is calculated as $\frac{360^\circ}{12}$. For example, at 2 o'clock, the angle between the hands is $2 \times 30^\circ = 60^\circ$.