The method of truth tables illustrated above is provably correct – the truth table for a tautology will end in a column with only "T", while the truth table for a sentence that is not a tautology will contain a row whose final column is "F", and the valuation corresponding to that row is a valuation that does not satisfy the sentence being tested. This method for verifying tautologies is an effective procedure, which means that given unlimited computational resources it can always be used to mechanistically determine whether a sentence is a tautology. This means, in particular, the set of tautologies over a fixed finite or countable alphabet is a decidable set.