The short story "The Cask of Amontillado" by Edgar Allan Poe is a tale of revenge. The narrator, a man named Montresor, lures his victim Fortunato deep into the family catacombs beneath his palazzo, chains him there, and bricks up the entrance to the chamber, effectively burying him alive. On the way through the vaults, Fortunato makes a "gesticulation," or gesture, that Montresor does not understand. Fortunato therefore assumes that Montresor is not a member of the brotherhood of the Masons. When Montresor insists that he is, he offers as proof the trowel that he carries.
The organization that Poe refers to in this passage is the brotherhood of the Freemasons or Masons. The Masons trace their origins back to the Middle Age guilds that regulated stonemasons who assisted in constructing cathedrals and other buildings. Freemasonry consists of progressive levels into which members are initiated or raised, and there are various signs, words, gestures, and grips that accompany each level. To become a Mason, men must be free, of good moral character, believe in a higher power, and be devoted to the brotherhood of men. Among the symbols used in Freemasonry are the traditional tools of stonemasons, which would include trowels.
The fact that Fortunato is a Mason tells readers something about his character. His acceptance into the order is indicative of his belief in God and generally good moral character, although his drunkenness is somewhat of a lapse. The fact that Montresor is not a real Mason may be one of the insults he refers to at the beginning of the story as motivation for his crime. His immorality may be responsible for the Masons rejecting him.
Fortunato knows that Montresor is not a real Mason because he does not respond to the "gesticulation" or "movement" that Fortunato makes, a secret signal that one member would give to another. In fact, Montresor calls the gesticulation "grotesque," which may be a bitter reaction to his exclusion from the order. Montresor insists that he is a Mason, and as proof he produces a trowel. In response, Fortunato says, "You jest," which means, "You are joking." Concerning Montresor, at least, Fortunato does not take the symbol of the trowel seriously. If he were not so drunk, he should have realized at that point that something was wrong, and he should have tried to escape.