When speaking the word 'history,' must we precede it with 'an,' as it is written, or may we precede it with 'a?' I rarely hear people say "An history of the world," but today somebody said "A history of the world," and it suddenly sounded wrong to me. Speaking of words beginning with 'h,' I wonder also how it came to be that some require 'an' and some require 'a.' I am curious about the difference, but I am more curious about how this came to be and why this is so. Also, why is it that some people pronounce some or all 'h' words with a 'y' sound? Thank you for your help.
To Scriber,

In speech, can certain "h" words sound more of less like they start with a vowel sound, either depending on the speaker or on the form of the word? For instance, "a history book" compared to "an historical remark" (where the accent on the second syllable of "historical" can make the "h" inaudible.
Typically "an" before a word beginning with "h" only happens when the "h" is silent ("hour", "honor") or when the first syllable of the "h"-word is unstressed. ("historical", "hysterical"). The first syllable of "history" is stressed, so "an" would not be used.

"a history of England"
"an historical novel which takes place in Spain"

Students: We have free audio pronunciation exercises.
Just remember than 'an' always precedes a vowel sound.
E.g. an hour (our), an elephant.

Notice that it doesn't matter if the next word begins with a vowel or not. Only the vowel sound matters.

On the other hand, 'a' always precedes non-vowel sounds.
E.g. a car, a unicorn (you-ni-corn)
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Thank you CalifJim.
That was the confirmation I was looking for because all I had was a gut feeling about it.

Could I ask for your view on my other post called "complicated punctuation"?

I found this in James Ellroy's "The Big Nowhere": 'Walking in, he thought he was entering a hallucination.'

Would that be an editorial slip?