I'm writing my thesis and need to write: "[...] and a/an I0 of [...]" and am puzzled about which indefinite article to use.

I(q) is a symbol for intensity as a function of the momentum transfer q and I assume the indefinite article before intensity would be an. I0 is the intensity where q=0 and is also known as the "forward scattering", and I would assume the forward scattering would have the indefinite article a.

So dependent on whether you see I0 as the forward scattering or the intensity at the point where q=0, I would use a different indefinite article. I read it in my head as forward scattering so the an "sounds wrong", but when I look at the page I think that the a "looks wrong". Are there any defined rules?

Thanks in advance and please bear with me as English is not my first language.

  • How it would be read, that determines a or an. May 14, 2018 at 13:08
  • How you read the expression in your own head is (in this and other matters) secondary to how your intended readers will read it in their heads. I would only prefer a over an if you were to do the translation from "I₀" to "forward scattering" in the text, as perhaps "and a forward-scattering value (I₀) of . . . ." May 14, 2018 at 13:11
  • @BrianDonovan Most authors don't have ready access to the heads of their intended readers. :)
    – Lawrence
    May 14, 2018 at 13:24
  • @Lawrence, I beg to differ. The task of the writer largely consists in imagining what readers will likely make of the text, and adjusting the text to adjust that effect. To take the point one step further, following Walter Ong, the intended readership is a fiction that the writer creates; and such being the case, the writer has full imaginative access to their heads. May 14, 2018 at 13:39
  • Your choice of a or an will provide a cue to your reader as to how you intend it to be read. Jun 13, 2018 at 20:05

1 Answer 1


It is not very common to refer to things like I0 with their meaning or definition in speech. People mostly just say "I-zero". Therefore: 'an'.

An I0 of

However, you could also write

A forward scattering I0 of

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