Watch four protractor readings on the IWB. We will work through them one at a time so you can see the placement and the reading on each.

Reading 35°: an acute angle

The centre of the protractor sits on the vertex. The zero is on one ray. The other ray crosses the inner scale at 35. Routine reading.

Say the routine aloud together: centre on vertex, zero on ray, read where the other ray crosses.

Reading 145°: the inner-vs-outer trick

Same placement, but now look at both numbers where the ray crosses. You can see 145 on the outer scale and 35 on the inner scale. Only one is right. Why 145 and not 35? Look at the opening of the angle: it is bigger than a right angle, so the answer has to be the bigger number.

Reading 90°: both scales agree

At exactly 90°, both scales read 90. The right angle is the only spot on the whole protractor where the two scales give the same number.

Hands up: anywhere else on the protractor where the two scales agree? (No, only at 90°.)

Reading 178°: almost a straight angle

The ray is squeezed right up against the straight line. Reading this one is fiddly: your eye has to land on the right tick mark, not the one next to it.

Today we explore: four new angles on the IWB protractor. Both scales are visible. For each one, one pupil comes up to the board and reads the angle aloud, saying which scale they used. The rest of us decide first: is the angle acute or obtuse? Then we check whether the reader picks the right scale.