Straight Angles – A Flat 180 Degrees
A straight angle measures exactly 180° – half of a full turn. Its two arms point in exactly opposite directions, so the angle looks like a perfectly flat, straight line rather than a “corner” at all. It's easy to overlook a straight line as an angle, but it's every bit as much a genuine angle as a sharp acute one.
A straight angle is exactly twice a right angle (2 × 90° = 180°), which is why it's the natural midpoint of the full 360° angle scale – everything smaller is acute, right, or obtuse, and everything larger (up to 360°) is reflex.
Recognising a Straight Angle
A straight angle measures exactly 180°. Its two arms form a single straight line, pointing in opposite directions from the vertex.
The two angles together make a straight angle: x = 180 − 130 = 50°.
Real-Life Application
- Flat surfaces: a perfectly flat tabletop or floor represents a straight angle.
- Fully opened books or laptops: opening all the way to lie flat forms a straight angle.
- Horizons: the horizon line, viewed edge-on, represents a straight angle.
Key Takeaways
- A straight angle is exactly 180°, and looks like a flat line.
- It is exactly twice a right angle.
- It marks the midpoint between the smaller angle types and reflex angles.
Practice: Straight Angles
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