Module 8 - Drawing Instruments
Parallel Rulers
What you will learn
Transfer a true direction between a plotted line and a chart compass rose without changing the angle, then resolve the intended direction from its reciprocal.
Parallel rulers preserve one line direction while they are walked across the chart. The technique succeeds only when one bar stays fixed during each movement and the final line is checked for both angle and sense.
Parallel rulers are two straightedges joined so their long edges remain parallel as they open and close. Align one edge with the course or bearing line, hold one bar firmly, move the other, then alternate until an edge reaches the compass rose or required chart point. If both bars move or the rulers slip, the original direction has been lost and must be set again.
At a compass rose, use the clearly labelled true graduation and make the ruler edge pass through the rose centre. IHO chart specifications state that bearings indicated on charts are true, but a printed rose may also carry magnetic information or more than one ring. Read its labels and chart notes; do not assume that an unlabelled outer or inner ring always has the same meaning on every chart.
One un-arrowed straight line has two directions separated by 180 degrees. A line set to 064 degrees also lies on 244 degrees. Before drawing or reading it, identify which way the vessel, bearing or line of position is intended to run, add the appropriate arrow or label, and write a three-figure direction with T, M or C as applicable.
Worked example
Plot a course of 064°T through a charted waypoint using parallel rulers.
- 1At the compass rose, align a long edge through the centre on the true 064°/244° line.
- 2Hold one bar fixed while moving the other; alternate the bars until an edge reaches the waypoint. If either bar slips, return to the rose and reset the angle.
- 3Draw a light line through the waypoint and add an arrow in the 064°T direction, not the 244°T reciprocal direction.
- 4Walk or place the rulers back to the rose and re-read the completed line independently.
Sense check: 064°T points generally north-east; an arrow generally south-west has selected the 244°T reciprocal.
| Check | Good evidence | Reason to reset |
|---|---|---|
| Alignment | Edge passes through the intended line or point and the compass-rose centre | Edge only touches a graduation away from the centre |
| Transfer | One bar remains fixed during each alternate movement | Both bars move, lift or visibly slip |
| Reference | The labelled true scale is used for a true charted direction | A ring is chosen only because it is inner or outer |
| Sense | Arrow, three-figure value and chart geography agree | The line points along the reciprocal direction |
Catch a reciprocal error before departure
A route card says 064°T, but the arrow on the charted leg points towards the south-western side of the chart.
1. Stop at the disagreement
Do not assume the arrow or number is correct merely because the line passes through the waypoint.
2. Re-read at the true scale
Transfer the line back to the labelled true compass-rose graduation and identify both reciprocal values.
3. Resolve the intended sense
Compare departure and destination, then place the arrow towards 064°T and label it explicitly.
Sense check: The numerical bearing, arrow and chart geography should all describe the same direction of travel.
Common mistake or limitation
- Moving both bars at once, lifting the rulers clear of the chart or continuing after a slip without resetting the original angle.
- Touching a compass-rose graduation without passing the ruler edge through the rose centre.
- Assuming outer means true and inner means magnetic without reading the actual rose labels and chart notes.
- Reading the correct line angle but selecting its 180-degree reciprocal as the intended course or bearing.
Recap
- Preserve the angle by holding one ruler fixed during each movement.
- Use the labelled scale and pass the edge through the compass-rose centre.
- Check the 180-degree reciprocal and label the intended direction and reference.
Optional quick check
Section 2 of 7
A course line is correctly aligned on 064°T/244°T but its arrow points south-west. What should you do?
Common questions
How do parallel rulers work?
Their joined bars remain parallel as they separate. By holding one bar fixed and moving the other alternately, you transfer the same line direction between a compass rose and another chart point.
Which compass-rose ring shows true bearings?
Use the ring or graduation explicitly labelled for true directions on the chart in front of you. Do not rely only on whether it is the inner or outer ring.
Why does a course line have two bearings?
A straight line can be followed in either direction. The two directions differ by 180 degrees, so the arrow, route endpoints and label identify the intended sense.
Keep revising this topic
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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