The International Regulations for the Prevention of Collisions at Sea, or IRPCS, were published by the International Maritime Organisation (IMO) in 1972, following a convention which created an international treaty. They are, effectively, the international marine equivalent of the Highway Code.
The code covers how vessels should react when at sea including how and where the regulations apply and how vessels should react when meeting each other at sea. The code covers what is required by seafarers when in sight of each other, not in sight of each other and when in restricted visibility.
Furthermore, vessels carry different day shapes and, at night or in poor visibility, light configurations / sound signals, which will help identify the type of vessel and its ranking in the IRPCS ‘pecking order’ when it comes to deciding which vessel is the stand on vessel and why.
The overriding principle of the IRPCS is that all vessels must stand a proper watch and avoid collision at all times. At sea, there are no rights of way. Simply definitions to determine which vessel should ‘stand on’ course and which should not. In all events both parties in a meeting are required to avoid an accident.
Whilst the IRPCS do not cover inland waterways, there is a general presumption that inland waterways will largely follow the same principles. Sailing vessels and motor vessels, from the largest to the smallest, are affected and all skippers should know the rules and abide by them. The International Sailing Rules set by ISAF largely mirror the IRPCS, although there are some important differences.
It’s worth noting that even when racing a sailor must adhere to the IRPCS when meeting vessels from outside the fleet, or all vessels after twilight.
The RYA Day Skipper and RYA Coastal Skipper shorebased courses teach the IRPCS in some detail and all candidates must sit an examination paper on this specific subject in addition to papers on chartwork and navigation.
More information can be found on the International Maritime Organisation website.
A leisure sailor should read the RYA book - RYA International Regulations for Preventing Collisions at Sea.