Plimsoll mark

The use of this symbol to denote standard thermodynamic quantities is then permissible only when these refer to the pure substance as standard state. Some other convention is needed to denote the more general standard quantities defined in equation (7.51), where the standard state may for example refer to an infinitely dilute solution. After much consideration the symbol has been introduced. This symbol is based on the circle, and is thus closely related to the more common (but occasionally confusing) notation for standard quantities. It seems useful, however, to associate it with the plimsoll mark which, appropriately enough, refers to a reference state of loading of a ship this use of an ideogram has, we believe, some value in keeping the essential nature of standard states in prominence. 

A superscript degree symbol (o) denotes a standard quantity or standard-state conditions. An alternative symbol for this purpose, used extensively outside the U.S., is a superscript Plimsoll mark ( ). ... 

The Plimsoll mark is named after the British merchant Samuel Plimsoll, at whose instigation Parliament passed an act in 1875 requiring the symbol to be placed on the hulls of cargo ships to indicate the maximum depth for safe loading. 

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