Key point A square corner is a right angle. If an angle opens less than the corner it is acute. If it opens more than the corner it is obtuse.

Four angles on the board, each drawn on a protractor. Take them one at a time and decide, before you read the label, whether it is smaller or bigger than a square corner.

The last two are the ones that matter. They sit either side of the corner by only a few degrees, so guessing by eye is not enough.

We take turns at the board while the class watches and agrees aloud. We take the cards one at a time: first a clearly narrow angle and a clearly wide one, then an exact square corner, and last one that is only just bigger than a right angle. For each card, decide whether the angle is smaller than, the same as, or bigger than a square corner.