This principle, dealt with in the ICZN as a sub-article of the Principle of Priority, is concerned with the preservation of stability and universality in zoological nomenclature. It operates to protect well-established names from being replaced by long-forgotten and hence unused senior synonyms, but can be modified by prevailing usage and the actions of the First Reviser. When the precedence of two names cannot be determined, that precedence can be fixed by the actions of the first author (the First Reviser) citing those names in a published work and selecting from them. Prevailing usage concerns the identification and possible valid publication of unused senior synonyms after 1899, and the variable conditions in which a junior synonym or homonym may take precedence, subject to a ruling of the International Commission on Zoological Nomenclature. Formerly, prevailing usage was not clearly articulated within the Code, and it is still subject to debate by those with differing views on the relative importance of stability versus priority.