Pietro Spoils Dom Gianni’s Mare Transformation is a comic tale of hospitality, credulity, and sexual deception, in which Pietro’s objection to a supposed magical tail ruins the transformation he has asked to witness. Dioneo introduces it as a warning about how a small failure to follow an enchanter’s conditions can undo his work; the scene’s irreverent action reveals that the spell is a cover for Dom Gianni’s sexual access to Gemmata.

A friendship shaped by hospitality

The story opens at Barletta, where Dom Gianni di Barolo, a priest with a poor living, supplements his income by buying and selling goods at fairs in Apulia. He travels with his mare and, on his journeys, forms a close friendship with Pietro da Tresanti, a fellow trader who travels with an ass. Dom Gianni calls him by the familiar title of Gossip Pietro, and when Pietro visits Barletta, the priest lodges and entertains him as well as he can.

Pietro reciprocates when Dom Gianni comes to Tresanti, though his household has little room to offer. Pietro lives in a poor little house with Gemmata, his young and attractive wife, and their ass; the couple have only one bed, which they share. The mare is stabled with the ass, and Dom Gianni sleeps beside her on a truss of straw. This awkward arrangement gives the story its practical pretext: Gemmata repeatedly offers to sleep at a neighbor Zita Caraprese’s house so that the guest can take her place in bed, but Dom Gianni refuses.

The promised transformation

To explain his refusal, Dom Gianni claims that he can turn his mare into a beautiful woman, sleep with her, and change her back whenever he wishes. Gemmata believes him. She tells Pietro that, if his friend is truly as close as he says, Pietro should learn the charm: then he could turn her into a mare and have dealings with both animals, supposedly gaining two for one. Pietro, described as somewhat dull-witted, accepts her reasoning and repeatedly presses Dom Gianni to teach him the trick.

Dom Gianni initially tries to cure Pietro of this folly. When persuasion fails, he agrees to perform the transformation the following morning before daybreak. He presents the tail as the most difficult part and insists that the couple must obey his directions if the operation is to succeed. The pair, who have scarcely slept from anticipation, rise early and summon him. Dom Gianni gives Pietro a light to hold and warns him not to speak, whatever he may hear or see, while telling him to pray that the tail will stick fast. Pietro promises to comply.

The interrupted spell

Dom Gianni has Gemmata strip and stand on all fours like a mare. He passes his hand over her face and head, naming them as a mare’s head and mane, then touches her arms and calls them legs and feet. At her firm, rounded breast, his arousal becomes apparent; he names that part a mare’s chest as well. He continues over her back, belly, rump, thighs, and legs, making the ritual’s bodily progression serve the story’s sexual double meaning.

Only the tail remains. Dom Gianni lifts his shirt, takes the dibble he uses for planting, and thrusts it into place, declaring that it is to be a fine mare’s tail. Pietro, who has watched closely, objects that he will not have a tail put there. The protest breaks the spell at its decisive instant. Dom Gianni withdraws the dibble and blames Pietro for speaking after being expressly told to keep silent: the mare, he insists, was already made, but the interruption has spoiled everything and the attempt cannot be repeated.

The joke’s aftermath and framing

Pietro defends his objection by saying he did not want a tail there and asking why Dom Gianni had not told him to put it on himself, adding that the priest had placed it too low. Gemmata, now standing up, rebukes her husband for ruining their supposed advantage and points out the obvious absurdity of a mare without a tail. Her credulity has not yet yielded to any recognition of the sexual ruse; within the story, she treats the failed spell as a genuine lost opportunity. She dresses, dejected, while Pietro returns to his former business with his ass and never again asks Dom Gianni to perform such a service.

Dioneo’s framing makes the tale’s comic mechanism explicit without resolving it into a sober moral. He tells his listeners to observe the conditions imposed by those who work enchantments, since even a slight mistake may spoil their effects. The audience laughs, and the narrator notes that the ladies understand the story better than he had wished. The contrast between the solemn rules of a magical demonstration and the plainly sexual act beneath them supplies the punchline: Pietro’s literal-minded interruption defeats not an enchantment, but Dom Gianni’s ruse. The tale closes among the day’s stories, after which the queen gives the crown to Pamfilo.