In 1400 the Gild of Cloth Merchants of Florence decided to make a thank-offering for the cessation of the plague; and the form which it took was a pair of bronze doors for the baptistery of the church of S. Giovanni, to correspond with some already there. These earlier ones are the work of Pisana and his son Nino, from designs by Giotto; the creation of the new ones was thrown open to competition. Many competitors appeared, of whom six were asked to submit specimens of designs for the panels; and, finally, when the choice lay between two only, the elder, Brunellesco, himself advised that the commission should be entrusted to Ghiberti, a youth then barely twenty years of age. The doors when completed contained twenty scenes from the Saviour's life, together with figures of the four Latin Doctors and the four Evangelists, set in a frame of exquisite foliage. This splendid work was surpassed by a second pair of doors subsequently made for the same place. In this there are ten panels setting forth scenes from the Old Testament history; and the frame is adorned with niches and medallions in which are placed some fifty allegorical figures and portrait heads. It was of these last doors, which were only completed in Ghiberti's mature age, that no less a judge than Michael Angelo said, "They might stand as the gates of Paradise itself." Aix-en-Provence claims that her doors are as peerless as examples of the wood-carver's art, as are the Florentine ones as types of the metal-worker's. They have been preserved, it is said, from the sixth century, and are still wonderfully fresh and delicate. There are on each door six upper panels filled with figures of the twelve Sybils; and below one large panel, occupied, in one case, by effigies of the prophets Isaiah and Jeremiah, and in the other by Ezekiel and Daniel. The carving is only occasionally exhibited, two masking doors having been cleverly contrived to protect and cover the real ones. Many of the doors of our cathedrals and great abbey churches have knockers, often of very striking designs. These as a rule indicate that the places in question claimed the right of sanctuary; and the knocker was to summon an attendant, or watcher, to admit the fugitive from justice at night, or at other times when the entrance was closed. A curious head holding a ring within its teeth forms the knocker at Durham cathedral; a lion's head was not an uncommon form for this to take, as at Adel, York (All Saints), and Norwich (S. Gregory's); a singularly ferocious lion's head knocker may be seen at Mayence.