The milagro's ancestry runs through the Iberian peninsula, where small votive offerings in metal and wax, promesas, were a fixture of Catholic shrine culture long before the conquest of the Americas. Spanish and Portuguese colonists carried the habit with them, and it took root wherever pilgrimage shrines grew: the great Marian basilicas of Mexico, the pilgrimage churches of the Andes, the santuarios of New Mexico, above all the little church of Chimayo, whose walls of abandoned crutches and pinned charms make the practice visible to every visitor. In many communities the milagro transaction has its own etiquette.
The charm may be promised in advance and delivered only after the favor is granted, or offered first as a standing reminder to the saint; some are worn or carried for a period before being surrendered. The same object can serve petition, thanksgiving and amulet in turn, which is why anthropologists resist filing it under any single heading. Where body-part votives elsewhere tend to be temporary accumulations, cleared periodically by shrine custodians and melted or buried, the milagro has also crossed into permanence: colonial-era silver examples survive in museum collections, and the form is reproduced as folk art worldwide. The living practice, though, remains what it always was, a small bright object standing for one household's trouble, left where the saint cannot overlook it.