grammar - Why use "an" before a word that starts with a "L"

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?

Answer

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.