This has always confused me. I've always spelled it "moustache", but my browser's spell checker claims the correct spelling is "mustache". From what I've seen around the Internet, people seem to use both.
What is the proper spelling? Are both acceptable?
Best Answer
This seems like a good one for Ngram.
It looks like in British English moustache has always been the preferred spelling.
In American English, it's been more of a battle with mustache now in the lead.
So here's a combined view. Both are contenders in the English-speaking world, but clear preferences exist on opposite sides of the pond.
(Have to wonder if the peaks and valleys correspond to mustaches going in and out of vogue.)