I've come across many sources claiming that the words firstly and first (as an adverb) can be used interchangeably. They, however, only seemed to be comparing their meanings with regard to enumerations - first(ly) I want to say ..., second(ly) ..., last(ly) ...

What bothers me more is if the word firstly is also usable within the meaning of at first or initially like in the sentence:

Muons (subatomic particles) were firstly observed in cosmic rays.

The word first fits there very well, but is that the case for firstly too? I have not found a dictionary definition or example saying so.

  • If you label the pragmatic (material-organising) marker usage of first / firstly as such, it simplifies matters. Then you can check the adverbial usages of first, say at AHDEL. Though admittedly they still have the dustbin adverb class. Which usage/s of first does it say overlap with that/those of firstly? Jul 5, 2015 at 19:23
  • Related issues are covered by “Firstly” or “first”? (and When enumerating motivations, is it correct to write firstly, secondly, …, or first, second, …? over on ELL), from which my impression is pedants and/or Americans don't like firstly so much even in contexts where I see little difference. But in OP's "strictly chronological" context here I think probably no-one would endorse it. Jul 5, 2015 at 19:32
  • Thank you for your remarks. I was really only lacking evidence that firstly is not used correctly in the cited sentence, since many sources rendered first and firstly equivalent. Jul 5, 2015 at 19:37

2 Answers 2


Firstly (see what I did there?): No. Firstly can't be used in the same meaning as at first or initially. As @deadrat mentioned, "firstly" meaning first is restricted to enumerations, so the initial observation of muons would be the first time.

Secondly, depending on the context of the text, the use of firstly in the excerpt you've given us may be correct after all (though with an incorrect word order). But I only consider this option because I read a lot of scientific papers and I know that grammar and writing isn't their (scientists) strong suit. I assume it should indeed be first, but I'll expand on my reasoning:

If the text is about how muons were originally observed for the first time ever, then yes, it should be first.

However, if the context of the text is different (i.e. it talks about how muons were observed in an experiment), then firstly could be correct. But only if it's followed by additional ways muons were observed. As I mentioned before, I only consider this because I know the grammar in a lot of scientific papers is horrendous. The only context I would accept firstly in would be:

"muons were observed firstly in cosmic rays, secondly in radioactive decay, [...]"

And that is assuming that the author of the text isn't great at grammar (which on a whole, they are not). The excerpt you've given is too short to be sure what the author meant.

So in short: firstly is only for enumerations and if the context of the paper meant it in that way, then the phrase you have shown us is gramatically incorrect (word order), but experience has taught me that in scientific papers, both of these options (incorrect use of firstly or just an incorrect word order) are possible.

PS: Thank you @FumbleFingers for pointing out I wasn't very clear in my reasoning behind my consideration of the second (though gramatically incorrect) option.

  • I don't recognise this distinction at all. To cook roast rabbit, firstly catch your rabbit, then pop it in the oven? I don't think so. Jul 5, 2015 at 19:36
  • @FumbleFingers I don't understand the analogy you're making here. Is it possible you misread the original post or the context I allow firstly to be used in here? Jul 5, 2015 at 19:43
  • @FumbleFingers Ah, I see what you mean. I included the part about firstly to show in which context of the text this would be allowed. But I absolutely agree it cannot mean the same as initially or at first . Thank you for pointing out it may be confusing, I'll correct that. Jul 5, 2015 at 19:46
  • I'm saying the same thing as @deadrat citing OED below - "firstly" meaning "first" is restricted to enumerations. I cannot see any way you can meaningfully justify firstly as being an acceptable way to distinguish one way of observing muons from another (we observed muons secondly using the LHC?). And I'm BrE, which I suspect makes me more tolerant in this area than most AmE speakers. Jul 5, 2015 at 19:50
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    @FumbleFingers I should have added my reasoning for even considering the second option. There's no question that phrase is gramatically incorrect. But the way in which it is incorrect may differ. Faulty use of firstly or just the word order. I do not know the rest of the text, so I included both options just to be sure. Reading a lot of scientific papers has taught me that it may very well just be an incorrect word order, which is why I wanted to include that possibility. I hope my edit has clarified that. Jul 5, 2015 at 20:14

Per the OED, "firstly" meaning first is restricted to enumerations, so the initial observation of muons would be the first time.


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