In this document, it says:

An LTI (Linear, Time-Invariant) system, in a simplified sense, will exhibit two behaviors

Can someone explain why the use of "An" instead "a" is correct here?

  • 1
    Say the letter "L"; do you say a "L", or an "L"? Commented Mar 13, 2017 at 22:45
  • 1
    Must be a duplicate...
    – Drew
    Commented Mar 13, 2017 at 23:11

2 Answers 2


The "a" v. "an" distinction is phonetically based. If you say L T I, when you pronounce the letter L is pronounced "el" (as in the proper name "Eleanor") which starts with a vowel.

If the acronym had been dispensed with, you would have used "A" instead.

  • Yeah, it's the sound that is significant, not the graphic symbol.
    – Hot Licks
    Commented Mar 13, 2017 at 22:51

Fowler says that it's a question of pronunciation. "Before letters standing for abbreviations or symbols the choice is usually determined by the sound of the letter, e.g. an R.A., an M. P." And I like how Fowler ends this line of thought: ". . . but that is the sort of thing about which we ought to be allowed to do as we please, so long as we are consistent." (H.W. Fowler, "A Dictionary of Modern English Usage," second edition, Oxford, 1965.)

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