Parallel Rules

A hinged chartwork instrument used to transfer a direction across a chart while keeping the same alignment.

The two straightedges remain parallel as the instrument is walked across the chart. This allows a course or bearing line to be moved to a compass rose for reading, or a direction from the rose to be transferred to a position or waypoint.

Accuracy depends on keeping the edges flat, preventing slip and using the correct charted compass reference. Parallel rules transfer direction only; they do not apply variation, deviation, leeway or tidal corrections automatically.