Poe's use of the word mason is one of the great examples of irony in this story. As Montresor leads him through the catacombs, Fortunato tosses a bottle upwards with a gesture Montresor does not understand. Seeing his lack of comprehension, Fortunato then says to him that he must not be a mason. Fortunato is referring to the freemasons, a society dedicated to fellowship in which the members help one another. Montresor, however, insists he is indeed a mason, and produces a trowel to prove his point. The irony is in the slippage between Fortunato's understanding of the term as one of brotherhood and benevolence and Montresor's understanding of his role as "mason": the one who will wall Fortunato up with bricks to die alone in the depths of the catacombs. Montresor as "mason" is clearly murderous and malevolent, but Fortunato, drunk and trusting, doesn't understand what sort of "mason" Montresor is until it is too late.