Constant bearing, decreasing range

The reasoning is subtle: non-sailors who are used to driving automobiles normally are able to detect the possible risk of a collision with implicit reference to the background (e.g., the street, the scenery, the landscape, etc.)

However, this method of recognizing non-CBDR conditions is only accurate for relatively small observing vessels.

One might think that another vessel which seems to be progressing from, perhaps, dead ahead to apparent movement down one's starboard side cannot result in a collision.

Because a vessel afloat can change its true heading (relative to north) without obvious detection (unless there are landmarks in sight) two vessels may collide even if one presents a substantially changing relative bearing.

At the crossing of two straight roads, a CBDR condition may occur and keep one vehicle in the other's blind spot.

Diagram showing principle of constant bearing, decreasing range in marine collision avoidance. When an observer sees another vessel at a constant bearing and the range continually decreases, collision is imminent.
A ship seen to be on a constant bearing with decreasing range will collide with the observer's ship unless avoiding action is taken.