This paper is primarily concerned with principles, not with practical implementation. It is therefore reasonable to start by constructing a simplified model of the physical situation in the hope that its analysis will provide guide lines for practical action.

Consider two vessels on a collision course in the open sea. An acceptable system of manoeuvres to ensure a safe passing should satisfy the following requirements;

(i) Each vessel must make a positive contribution to the safety of the encounter.

(ii) A mariner must be able to decide on his course of action on the basis of information readily available to him, i.e. on the compass bearing of the threat, not on its true motion or aspect.

(iii) All vessels must be able to operate in accordance with the same set of rules, i.e. use a standard diagram showing the prescribed manoeuvres as a function of compass bearing only.

(iv) The same set of rules must apply whether the two vessels are in visual or in radar contact.