An angle measures the turn between two rays that meet at a vertex. A full turn is 360°. The size of the marked turn determines the type of angle.
Acute angle
Greater than 0° and less than 90°
A diagonal drawn from the corner of a square makes a 45° angle with its edge.
Right angle
Exactly 90°
A right angle is one quarter of a full turn, like the corner of a square.
Obtuse angle
Greater than 90° and less than 180°
A 120° angle turns 30° farther than a right angle.
Straight angle
Exactly 180°
The rays point in opposite directions. Together they make a straight line.
Reflex angle
Greater than 180° and less than 360°
If the smaller gap is 90° and the arc takes the longer path, the marked turn is 360° − 90° = 270°.
Which part of the diagram should you measure?
Follow the gold arc around the vertex. The same rays can mark a smaller and a larger turn, so check which one the arc follows. Try the daily angle guessing game or compare two angles to practice.