Which is correct, twelveth or twelfth?

The correct spelling is twelfth. It refers to the ordinal number after eleventh and represents the 12th position in a sequence. Many people get confused with the spelling of numbers in English, but "twelfth" is the accurate form. In English, numbers like first, second, third, etc., follow a unique pattern. "Twelfth" fits into this pattern, with "th" at the end instead of "eth," which is a common mistake when writing numbers in their ordinal form.