A parallel calliper is also named a jaws, and consists of a sort of el-square, on the blade of which slides one of the jaws of the instrument. On the blade are two or three rows of centimetres and millimetres, and also inches and parts; and by means of the measures the jaws are put near each other, or separated to any distance that can be indicated by the tool. The sliding jaw is provided with a small fixing screw to tighten it at any desired place along the blade. A jaws is indicated by Fig. 421, and the tool is useful to directly measure the thickness of any object which is between the jaws, such thickness being known by inspecting the measure, and without reference to any other standard. Measuring by such an instrument is sufficiently near to correctness for a great variety of work, both for lathes and vices; consequently the tool is preferable to callipers having curved legs when such measuring is desirable, because the thickness of any piece of work is indicated by one tool instead of two. The tool shown by Fig. 422 resembles a parallel calliper which is deficient of the sliding jaw. This instrument is a butt measure, or a stop measure, and consists of a straight-edge which is accurately marked with centimetres and millimetres, and having at one end a stop which is at right angles to the length of the straight-edge. While the tool is in use, the work to be measured is put to the instrument, so that one extremity shall touch the inner side of the stop, and while thus held the measurement can be accurately performed, without any need of looking to see if the extremity of the work coincides with the extremity of the measure.