grammar - "All right" vs. "alright"

Is it “alright” or “allright”?

Which is correct in English, "all right" or "alright"?

These expressions don't cause any problem in verbal communication but I confuse them while writing. To be frank, I sometimes use "all right" and sometimes "alright" without actually knowing which one is correct. If both of them are correct, how should I know which one to use in a particular case?