This slide rule is a circular version of those used for over one hundred years for performing multiplication, division and square roots for many technical applications. It had scales for basic multiplication and trig. functions. The scales did not move or slide against each other, which is uncommon.

Slide rules were very common until the mid-1970s, when they were all but replaced by electronic calculators. They came in many configurations, including models that had scales specialized for a particular engineering specialty, including aerospace. Aircraft crews often preferred the circular style, as it could be more easily manipulated with one hand.

This rule was found in the collection, stored with miscellaneous aviation materials of a variety of origins.