1. This was one of range of circular rules made by this British company. It is designed specifically for trigonometric calculations.

2. The rule has a single cursor on the front and a second independent cursor on the back.

3. The rule has a fixed inner and outer disc and rotting central disc which is rotated by means of clear plastic cover.

4. The scales on the back are independent of those on the front. Summary instructions are given on the back.

5. For a copy of the manufacturers instructions click here..