What measuring instrument did Leonardo da Vinci design to draw proportional circles and angles?

Leonardo da Vinci designed a proportional compass to draw proportional circles and angles.

A proportional compass, also called a sector, uses hinged arms and marked scales to transfer measurements at chosen ratios. By setting the instrument to a particular proportion, a user can enlarge or reduce a drawing while preserving its relationships. It can also help with geometric constructions and calculations.

Leonardo studied and drew versions of proportional compasses during his investigations of geometry, architecture, and engineering. The instrument was especially useful to artists and designers who needed to scale plans or compare dimensions. It was not a compass for finding magnetic north.

Leonardo was not the only Renaissance thinker interested in this class of tool, and later versions were developed and used by engineers, navigators, and mathematicians. The important distinction is between a proportional compass and an ordinary drawing compass: the ordinary type draws circles from a fixed point, while the proportional instrument works with ratios between measurements.