Multiple comparisons in a mixed model

In this note we give a simple proof of the result that, in the symmetric version of Scheffe's mixed model for a balanced two-way layout, an exact T procedure for pair-wise comparisons between the levels of the fixed factor can be based on the interaction mean square as the Studentizing factor. Such a proof does not seem to be available in the literature and will hopefully remove the confusion caused by some textbooks that incorrectly prescribe the error mean square as the Studentizing factor.