A liáng jiǎo qì (protractor) is a semicircular or circular measuring tool used to measure or draw angles in degrees. The word breaks down literally as 'measure' (量), 'angle' (角), and 'tool/instrument' (器), making its meaning highly transparent: 'an instrument for measuring angles.' It is commonly made of transparent plastic or metal and features degree markings from 0° to 180° (or 360° for full-circle versions).

This word appears frequently in school settings—especially in mathematics and geometry classes—where students use it to construct or verify angles in diagrams, maps, or engineering sketches. It’s also used in technical drawing, carpentry, and navigation. Unlike general measuring tools like rulers or tape measures, the protractor is specialized for angular measurement only, and its name reflects that precise functional purpose.