Module 8 - Drawing Instruments
Parallel Rulers
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.
| 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 |
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.
Optional quick check
Section 2 of 4
A course line is correctly aligned on 064°T/244°T but its arrow points south-west. What should you do?
Keep revising this topic
Sources and factual review
Reviewed 20 August 2026 - Day Skipper Revision.
Continue studying Drawing Instruments
This public lesson is part of Revision Module 8. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.