The 'true' sextant measures an angle of 120 degrees. Sextants are invariable made of metal (or plastic) and can be of the 'drum' or 'vernier' type where it comes to the fine reading. The drum type uses a scale graduated in whole degrees and a wormgear to measure within the minute. The vernier type uses a scale along which a finely graduated scale slides. They are usually less accurate than the drum type.