1. This rule was made by a little known company in England, possibly shortly before slide rules ceased to be made. It is in three layers. The lower layer which has the three black scales, the clear plastic layer which is linked to the rotating red scale and the cursor. The rule is 75 mm (3 inches) in diameter.
2. The cursor has the names of the scales marked on it - which helps to avoid a problem with circular rules - knowing which scale you should be looking at.
3. The cursor also has an additional hair line, at Ö (4/p), whose use is not explained in the instructions.
