The custom was firmly established in Rome itself, and Pope Innocent III, by a letter of 18 January 1200, grants to the canons of St. Peter's the monopoly of casting and selling those "signs of lead or pewter impressed with the image of the Apostles Peter and Paul with which those who visit their thresholds ["limina"] adorn themselves for the increase of their own devotion and in testimony of the journey which they have accomplished". The pope's language implies that this custom had existed for some time. In form and fashion these pilgrims' signs are various. From about the twelfth century the casting of these devotional objects continued until the close of the Middle Ages and even later, but in the sixteenth or seventeenth century they began to be replaced by medals properly so called in bronze or in silver, often with much greater pretensions to artistic execution.