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Rule 1 is the first gate: establish the water, authority and applicable rules before relying on a remembered International, inland or local answer.

Use WATER — AUTHORITY — RULE SET — SPECIAL RULE — VESSEL — VISIBILITY — RISK. Rule scope and national implementation must be established before an encounter rule is applied. The International Rules apply on the high seas and on waters connected with the high seas that are navigable by seagoing vessels, while administrations may make closely conforming special rules for roadsteads, harbours, rivers, lakes and connected inland waterways. The water and competent authority therefore matter as much as the diagram in a revision question.

A national implementation, inland boundary, harbour direction, traffic service or local rule can change the text or add an operational requirement. Do not infer the boundary from appearance alone. Check the current official chart, publication, notices and authority material for the actual waterway, and distinguish an International-rule answer from a jurisdiction-specific answer in the brief.