The longitudinal chronometer (also called a marine chronometer) was a very precise clock that was used to measured time accurately. It allowed a ship's longitude to be determined by comparing the time shown to the local time, which could be worked out using mathematical and astronomical calculations. Its 3-level design, made of wood, allowed the dial to be easily seen when the case was locked. The dial has 3 concentric circles. The two inner ones are marked with numbers written in Roman numerals. The outer circle is for minutes and has unnumbered lines. The second hand has an independent dial on number VI.