Dividers measure distance on a chart: span two points, then read the gap off the latitude scale, where one minute of arc equals one nautical mile. The traditional one-hand pattern opens and closes with a single hand, leaving the other free for the chart.
Here you will find brass one-hand dividers in 7 and 8 inch, a straight divider with adjustable tension, the unusual ten-point divider and a long brass pencil compass for range circles.