When Brunelleschi's dome was being constructed from 1419 to 1936 wooden scaffolding was erected on the side where the Porta della Mandorla opens out onto the adjacent houses, where via dei Servi begins today. In one of these homes lived a young and beautiful woman, a seamstress, who used to look out of the window at the men as they worked. She started to exchange glances with one of them and they soon began to meet secretly in her house when the young woman's husband was away at work. Besides the damage done, the adulterous worker wanted to leave a sign on the Duomo of the affair: he created a cow's head with two horns and put it on the Duomo, facing the direction of the women's home. It was meant for his lover's husband, who was "cornuto", meaning a man who was cheated on by their spouse.
Another reason for the cow's head was because, at the time, cows would transport the material needed to construct the Duomo along via dei Servi. As a sign of gratitude towards these animals that worked alongside the builders, they erected and attached the cow's head to the cathedral.