This research refers to the study of an arch adjoining two rooms of the Pio Clementino Museum, in the Vatican (Rome). This arch was designed by the architect Michelangelo Simonetti (1724–1781) around 1780. The warped intrados of the arch is defined by a flat round arch placed on the wall of the square vestibule and by a hunched curve traced on the cylindrical surface of the next vestibule. The resulting warped surface is a portion of a ruled surface that can be considered a good example of the lathe art and multi-axial modeling. The digital clone analysis demonstrated a double kind of perspective: structural and optical. The first was influenced by the researches on the stereotomic connections of ruled surfaces explained in treatises published in the second half of the 18th century. The second reference can be identified with optical-perspective studies applied to correction of sight like the pictorial decoration by Father Andrea Pozzo (1642–1709) which is present in the Casa Professa’s Chapel (Rome).
