text: At the time of development in the nineteenth century, the superscript Plimsoll symbol () was adopted to indicate the non-zero nature of the standard state. IUPAC recommends in the 3rd edition of Quantities, Units and Symbols in Physical Chemistry a symbol which seems to be a degree sign (°) as a substitute for the plimsoll mark. In the very same publication the plimsoll mark appears to be constructed by combining a horizontal stroke with a degree sign. A range of similar symbols are used in the literature: a stroked lowercase letter O () or a circle with a horizontal bar either where the bar extends beyond the boundaries of the circle () or is enclosed by the circle, dividing the circle in half (). Compared to the plimsoll symbol used in 1800s text, the U+29B5 glyph is too large and its horizontal line does not sufficiently extend beyond the boundaries of the circle. It is easily confused with the Greek letter theta (uppercase Θ or , lowercase θ ). As of 2024, the character  has been proposed for Unicode. It is a regular-sized Unicode symbol meant to be used in superscripted form when denoting standard state, replacing U+29B5 for this purpose.
  Ian M. Mills, who was involved in producing a revision of Quantities, Units and Symbols in Physical Chemistry, suggested that a superscript zero () is an equal alternative to indicate "standard state", though a degree symbol (°) is used in the same article. The degree symbol has come into widespread use in general, inorganic, and physical chemistry textbooks in recent years. When read out loud, the symbol is pronounced "naught".