One of the mysteries of the English language finally explained.
Why do we use ‘first, second,..’ and not ‘firstly, secondly...’?
When listing things there is a convention that the first item is introduced as first rather than firstly, although the following items can be said to be secondly, thirdly, fourthly, etc.
The reason for this is that first early on had a role as an adverb, and the use of firstly, though established by the 17c, was felt to be an unnecessary affectation. Today this rule is considered outdated, and a variety of sequences are sometimes used:
First…, secondly…, thirdly…
Firstly…, secondly…, thirdly…
Firstly…, second…, third…
The first two of these examples are both usually considered acceptable, but the third should normally be avoided.
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