What Renaissance measuring instrument did Leonardo da Vinci use in studies of geometry and perspective?

Leonardo da Vinci used and studied a proportional compass for geometric measurement and construction.

A proportional compass, also called a sector, has hinged arms marked with scales. By setting a ratio between the scales, a user can enlarge or reduce measurements while drawing. This made the instrument useful for geometry, architecture, surveying, mapmaking, and perspective work.

The proportional compass was not exclusively Leonardo’s invention. Similar instruments were developed and refined by several mathematicians and engineers during the Renaissance. Leonardo’s notebooks contain detailed investigations of geometry and mechanical tools, showing how such instruments could support practical design.

It is important not to confuse the proportional compass with a magnetic compass. A magnetic compass indicates direction using Earth’s magnetic field, while a proportional compass compares lengths and transfers a chosen scale. Leonardo’s engineering depended heavily on this broader tradition of measurement and geometric construction.