Fig. 1: A vernier callipers. The main scale is in millimetres, with the 3 centimetre mark at 30 mm and the 4 centimetre mark at 40 mm. The vernier scale has 10 divisions; its zero lies between the 31 and 32 mm marks. A red arrow marks the vernier line that lines up exactly with a main scale line: it is the 7th vernier line, level with the 38 mm mark.