The Rule of Thirds has four scales graduated in 1/3", 1/6", 1/9" and 1/12" increments. It allows you to make measurements in these dimensions and to easily divide dimensions into thirds, sixths, ninths and twelfths. The layout of the Rule of Thirds minimizes the need to perform mental and scratch paper calculations to achieve the results you need for your craft and woodworking projects. E.g. there is a scale graduated in absolute thirds-of-an-inch from 1/3" to 36/3" - alleviating the need to reduce complex fractions to compound fractions (e.g. from 17/3" to 5 & 2/3" form - simply read 17/3" right off of the Rule of Thirds. The Rule of Thirds was designed with scale modlers in mind. It has a direct reading 1 Foot = 1/3" scale which can be used for any other primary scale dimension as well - e.g. 1 Inch = 1/3", 1 Meter = 1/3", 1 Mile = 1/3".