Right angles around us
The right angle is a special angle , exactly a quarter turn, or 90°. It is the favourite angle of carpenters, masons, and book designers. Once you can spot a right angle, you start seeing them everywhere.
What is this?
A right angle is formed where two arms meet so that one is upright and the other is flat , like the corner of your textbook, or the meeting of the wall and the floor.
We usually mark a right angle with a small square drawn at the corner, like ⌐. That little square is a "name tag" saying, "This corner is exactly 90°."
A right angle is so important that we use it to compare other angles. If an angle is smaller than a right angle, it is acute. If it is bigger, it is obtuse.
Look around. The four corners of your slate are right angles. The edges of a door make right angles with the floor. The intersection of two streets often makes right angles. Even the screen of a phone has four right-angled corners.
A very neat trick: take a square sheet of paper. Fold it in half so that two opposite edges meet. Open it. Now fold again at right angles to the first fold. The crease lines cross at , yes , right angles. The corner of any folded paper is a perfect right angle, and you can carry it around as a checker.
Worked examples
Example 1. Look at the corner of a chalkboard. Is it a right angle?
- The two edges meet so that one is horizontal, the other vertical.
- That makes a quarter turn between them.
- Yes, it is a right angle (90°).
Example 2. How many right angles does a rectangle have?
- A rectangle has 4 corners, all square.
- 4 right angles.
Example 3. From 12 to 3 on a clock face, how many right angles has the minute hand turned?
- 12 to 3 is a quarter turn = 1 right angle.
Try it
- Name 3 objects in your room with right angles.
- How many right angles in a square?
- From 12 to 9 (going forward), how many right angles does the minute hand turn?
- Are all four corners of an open book right angles?
- Draw a right angle in your notebook and mark it with the small square symbol.
Hands-on
Paper-corner checker. Take any rectangular piece of paper. Its corners are right angles. Use one corner as a "tester" , hold it against different corners in your house and check which ones are right angles.
Right-angle hunt. Walk through your home for two minutes. Make a list of every right angle you see. Doorframes, windows, picture frames, tiles , count them all.