Grammar and Punctuation |
The Blue Book of Grammar and Punctuation

Secondly, Thirdly, Fourthly

As noted earlier, few people say "firstly," and fewer yet say "fifthly," "sixthly," "seventeenthly," etc. Many adverbs do not end in -ly. It makes more sense to use second, third, and fourth rather than secondly, thirdly, and fourthly.

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