Old and New Plurals
The plurals of some words often depend on whether they are used in a scholarly work or in more casual writing. Examples of these (noting the informal word first) are appendixes and appendices, memorandums and memoranda, millenniums and millennia, symposiums and symposia. Most dictionaries list the less formal as the preferred usage. The same word may have different plurals depending on its definition. A book could have two indexes while a mathematical expression might have two indices.
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