Could anyone explain the differences between "thru" and "through"? Is the difference only in spelling? Is "thru" some sort of slang?
I was going to comment that this is General Reference, but actually looking for a reference didn't come up with much.
Merriam-Webster has this...
Wiktionary notes the following, which gives more details on the "technical" usage:
So yes, thru is some sort of slang or jargon.
In Strunk & White, E.B. White's answer was:
White quotes the original edition, in which Strunk said,
White adds, somewhat wistfully,
(The Elements of Style, 1979 edition, p. 74-75)
Yes, “thru” is incorrect, but it's shorter to write. Just like “u” for “you” etc.