Correct spelling


Correct spelling, explanation: twelfth is an ordinal number from twelve. In twelve there is a consonant v, so it may seem, that after adding ordinal suffix -th, the form would be twelvth. However we can’t do it due to pronunciation. T in the suffix is an unvoiced consonant, so a consonant directly before must be unvoiced as well. F and v are technically the same sound (the former is unvoiced and the latter is voiced), so they are changed with pronunciation. This is why we spell this word twelfth.

Definition of twelfth:
numeral, ordinal number between eleventh and thirteenth.
It’s Julie’s twelfth birthday next week.
We were twelfth in line for the restaurant yesterday, I thought we would never sit, but it went smoothly.

Incorrect spelling


Incorrect spelling, explanation: twelvth is a wrong form, because v is a voiced consonant and t is an unvoiced one, so they can’t occur one after another. Because of the fact, that v and f are linguistically speaking a same sound (but f is unvoiced), for better pronunciation it was changed in twelfth. The same happens for example in five and fifth. Therefore twelfth is the correct spelling.