Question: I was wondering if "An" should be used with *obligatory* or "A". I understand that since *obligatory* begins with a vowel I should use "An" but I see people are using "A" more frequently than "An"

To me "A obligatory like" sounds right, as in "a Facebook like".

Also .. Is "and a obligatory comment" correct or is it "and an obligatory comment" ?

Answer: 
To me only "an obligatory" sounds right. "a obligatory" is not only more difficult to pronounce but also sounds pretty odd (to me at least). [Google ngram](https://books.google.com/ngrams/graph?content=an%20obligatory%2C%20a%20obligatory&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Can%20obligatory%3B%2Cc0%3B.t1%3B%2Ca%20obligatory%3B%2Cc0) seems to agree with me:

![Google Ngram](https://i.stack.imgur.com/HH7xb.png)
The indefinite article "a" changes to "an" when used before a word that begins with a, e, i, o, or short u, ignoring any silent consonants.

Use "a" for long u, whether it's spelled "eu" or just with a "u."

Only use "an" for words that begin with "y" if it's with an initial "y" that sounds like a short i. Words like this are very rare, nonetheless, listen to this [example](https://www.youtube.com/watch?v=x8oYgabMdIQ). (an ytterbium element)

Use "a" for *w* and phrases beginning with *one/once*

> 
> an honest mistake
> 
> 
> a universal trait
> 
> 
> an ugly duckling
> 
> 
> a euphoric feeling
> 
> 
> a one-time pad
> 
> 
> an only child
> 
> 
> a yes-man
> 
> 
> an iatrophoic person
> 
> 
> an idiot
> 
> 
> a wish upon a star
> 
> 
> 

So, *an obligatory ...* etc.