Every collision situation implies the existence of a sector within which any course steered will lead to a dangerously close approach and a sector within which the danger of collision will be avoided. The definition of these sectors may be said to constitute the structural aspect of the collision problem. The appropriate manœuvre in a given situation constitutes the operational aspect. The structural aspect is fundamental to the problem itself, the operational aspect offers possible solutions. The Steering and Sailing Rules involve the structural aspect because this is the only basis for legal rules.

In proposing a Sector Rule for use with radar in conditions of bad visibility, it is intended to establish a structural basis for legislation. In its most general terms, the Sector Rule is based on the principle that if the radar echo of another ship is always at an angle to one's own course determined by the speeds of the two vessels, their distance apart and the safe passing distance, then one's course lies within the safe sector for collision avoidance.