Fig. 1: A screw gauge. On the sleeve, whole millimetre marks 0, 1, 2 and 3 are above the datum line, and half millimetre marks are below it; the half mark after 3 is the last one showing before the thimble. On the thimble's circular scale, division 18 is on the datum line (between 15 and 20, three small marks past 15).