Marquois scales

[3] Quick construction of precise parallel lines was useful in cartography and engineering (especially before the availability of graph paper) and Marquois scales were convenient in some challenging environments where larger equipment like a drawing board and T-square was impractical, such as field survey work and classrooms.

[4] Marquois scales fell out of favour among draftsmen in the early 20th century,[5] although familiarity with their use was an entry requirement for the Royal Military Academy at Woolwich around the same time.

The triangle would be used for many regular set square operations, the rulers likewise would function as rulers, but the unique function was the 3:1 reduction ratio between measured distance and drawn line.

A similar method also allows the distance between points on plans or maps to be measured with increased precision.

Likewise the scales on other rulers are usually intended to be used directly and the selection differs accordingly.