Parallel Rulers

Parallel rulers are two hinged rulers that can 'walk' across the chart while maintaining a constant angle. Place the ruler along the line whose bearing you want to measure, then walk it to the nearest compass rose, keeping one ruler firmly pressed while moving the other. Read the bearing from the outer (True) ring.

They take practice to use accurately — the key is pressing firmly on one ruler while sliding the other. On a moving boat, this can be challenging. Some people prefer proprietary plotters for ease of use.