Angles, symmetry and the properties of shapes

Naming angles

A right angle is $90^\circ$. Less than that is acute, between $90^\circ$ and $180^\circ$ is obtuse, exactly $180^\circ$ is a straight line. Estimating first — “this looks a bit less than a right angle” — stops most protractor errors.

Using a protractor

Put the centre on the vertex and one arm along the zero line. A protractor has two scales; use the one that starts at $0$ on your arm. This is why estimating first matters: if you estimated acute and read $150^\circ$, you used the wrong scale.

Symmetry

A line of symmetry folds a shape onto itself exactly. A square has four, a rectangle two, an equilateral triangle three, a circle infinitely many. A parallelogram that is not a rectangle has none — a surprise that Checkpoint asks about often.

Shape families

A square is also a rectangle, and a rectangle is also a quadrilateral. Statements like “all squares are rectangles” are true; the reverse is not.