But what exactly Et Al. is short for depends on how strict you want to be with your Latin grammar.
The "Al." element is an abbreviation of the adjective alius, alia, alium (other, another).
If you search on hundred year old uses of the word pair, you find that in full bibliographic usage, it was not ordinarily abbreviated at all. And the writer then was required to use the correct number, gender, and case for the context in question.
If quoting a mere list of authors, the nominative plural would be correct, e.g.:
Smith, John, et alii
If, by some chance, the other authors were all female, the nominative plural feminine would be required:
Smith, Mary, et aliae
Very frequently, however, the context of the "Et Al." was in the accusative. So, for instance, if you told your reader to see a particular source, you could write:
Videte John Smith et alios.
In legal Latin, reports of charges brought against multiple defendants would similarly use the accusative plural:
William Parker filed a case against John Smith et alios.
"Et Alios," regardless of the correct grammatical context, ossified as the default full quote, but to avoid being grammatically incorrect, it is best to stick to the actual abbreviation, "Et Al."
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