The Plimsoll line (International Load Line).

The Plimsoll line is a mark on the side of vessels to indicate the maximum permissible loading depth. It was introduced in Great Britain through the Merchant Shipping Act of 1875 and soon found worldwide acceptance. It is named after Samuel Plimsoll, a shipping merchant who campaigned for its introduction.

The maximum safe loading depth varies with ocean regions and seasons. In the tropics the water is warmer and therefore less dense than in temperate regions, so with the same load ships will float higher in cold regions than in the tropics. Summer and winter cause similar changes. The Plimsoll line therefore contains several lines, with letters to indicate cargo, season and location (differentiated on the basis of local salt content of the water):

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