Proportional Divider - Gentle Lioness In-Depth Tutorial

This is a proportional divider. 
Proportional dividers allow architects and artists to transfer and scale up or down a line drawing to another piece of paper, while keeping the same proportions. Use the sliding thumb nut located in the center of the proportional dividers to pick the scale you want to draw in and begin sketching your accurately scaled illustration.
Watch a video showing how to use this tool here.