The 12th-century baptistery sits beside the cathedral on Piazza Arringo, an octagonal Romanesque structure of plain travertine raised over an earlier Christian foundation. The exterior is sober and undecorated; the interior preserves an octagonal immersion font and traces of frescoes. Both are typical of Marche Romanesque at its most disciplined.
Free to enter when open, which is usually during cathedral opening hours. Five minutes' worth of looking inside; the building is more impressive from the square.