This Tool of The Trade is of course a pair of dividers, always to be found on or near the chart table in a submarine's control room. This handsome example is a replica of the style used by the Ships Master on board HMS Association, which was wrecked off the Scilly Isles on October 22nd, 1707.
The dividers in this photo are resting on a battered chart of the North Pacific,used during a cruise to Hawaii in 1980. The ketch Wise Eye made a safe but prolonged passage, but that's a longer story for another time.
Below, the dividers are used to measure the distance between two point on the chart; one minute of latitude equals one nautical mile.