Definition
Protractor
A protractor is a degree-marked tool used to measure or draw angles.
What it means
Think of a protractor as a turn reader. Its centre sits exactly on the angle’s corner, and its straight baseline lies along one side of the angle. Start at the 0° on that side, then follow the numbers until the other side crosses the scale. Starting from the opposite zero gives the other reading, not the angle you set up.
Remember it
Centre on the corner, baseline on one side, zero on that side, then read.
Example
Mina places the centre mark on the vertex and lines the baseline up with the right-hand ray. That ray begins at 0° on the inner scale. The other ray crosses the 65° tick, so the angle measures 65°.
Watch for
A protractor measures the amount of turn between two rays; it does not measure the rays’ lengths.
Common mix-up
Do not choose a scale by its colour or position. Use the scale whose 0° is on the ray aligned with the baseline.
Exact meaning
A protractor is a semicircular or circular measuring instrument with a centre mark, baseline, and degree scale. To measure an angle, place the centre mark on the vertex, align the baseline with one ray, and read the degree value reached by the other ray on the scale that starts at 0° along the aligned ray.