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You already know a right angle: a square corner, like the corner of this page. Today a kite plan uses little marks instead of words.
One corner of the plan has a small square drawn in it. What does that mark tell you?
Geometric diagram conventions
Use and interpret standard geometric diagram conventions: mark right angles with a small square, equal lengths with single or double tick marks, and equal angles with arc marks; label angles in three-letter notation (∠ABC) and individual angles with a single letter or number; draw diagrams showing angles at a point, angles on a straight line, and angles inside polygons with these conventions; read diagrams with these marks to identify given information and find unknown values
What a learner can do afterwards
- Mark a right angle with a small square symbol in a diagram and explain what it means
- Use tick marks to show equal lengths in a shape and double tick marks for a second pair of equal sides
- Read and interpret angle notation (e.g. angle ABC) and identify the angle being referred to in a diagram
1 · Read
Geometry diagrams use small marks so you don't have to measure everything yourself. Once you know what a mark means, you can read the diagram like a set of clues.
A small square drawn in a corner always means that angle is a right angle, exactly 90 degrees. It does not matter how the corner looks in the drawing, the square tells you the true value.
Try it together
Look at a triangle with a single tick mark on two of its sides. Those two sides are equal in length. If a third side has two tick marks instead, it belongs to a different equal pair, not the same pair as the single ticks.
Angles get names too, using three letters, like angle ABC. The middle letter, B, tells you where the corner is. The other two letters name the two lines that meet there.
Two angles that share the same curved arc mark are equal. That is how a diagram hands you an angle without a protractor: find the marked pair, work out one of them from the angles you already know, and the other one matches. Marks are given information, so read them before you measure anything.
A square means 90 degrees, matching tick marks or arcs mean equal, and the middle letter in a name like angle ABC is always the corner.
2 · Watch
3 · Play
Take it off screen
Print a worksheetA4 with an answer key page for grown-ups. No screen, no internet.
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
Then practise
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.