Convention and protocol describe two different stages in how IMO treaties actually work — a convention is the original agreement, and a protocol amends it later without starting over.
Convention: The Original Treaty
A convention is the primary international agreement establishing a comprehensive set of rules on a given subject — SOLAS (Safety of Life at Sea), MARPOL (pollution prevention), and STCW (training and certification standards) are all conventions, each representing the foundational international agreement in its subject area.
Protocol: An Amendment, Not a New Treaty
A protocol is a subsequent legal instrument that modifies, updates, or adds to an existing convention — without replacing the entire original document. Protocols exist specifically because renegotiating an entire convention from scratch every time updates are needed would be impractical; a protocol lets the international community amend specific provisions while the bulk of the original convention remains in force.
The Clearest Real Example: MARPOL 73/78
MARPOL was originally adopted in 1973, but never entered into force in its original form due to insufficient ratifications. The 1978 Protocol was adopted to address this — incorporating the original 1973 Convention with modifications, and this combined instrument (referred to as “MARPOL 73/78”) is what actually entered into force and remains the operative framework today. This is exactly why you see the treaty referenced with two years — the original convention year and the protocol year that made it operative.
Why Understanding This Matters Practically
When you see a convention referenced with a protocol (MARPOL 73/78, or other IMO instruments with subsequent protocols), it tells you the current legal instrument is the combination of the original convention plus the protocol’s amendments — not two separate, independent treaties, and not simply the original unmodified convention either.
Exam Relevance
Understanding convention vs protocol, and correctly explaining why MARPOL is referenced as “73/78,” is a standard Ship Knowledge topic in deck and engineering officer competency exams.