This rule was made by Gilson but sold by other companies also, including Dietzgen, POST and others.

Most famous circular slide rule, and most useful in the USA.

Include fractional converter and very precision trigonmetric function.

About twice precision compared to the 25cm type rules:
Ex. Binary scale shows 1.005~9.98, while C scale of 25cm type shows 1.01~9.95.