Planetary symbols are used in astrology and traditionally in astronomy to represent a classical planet or one of the modern planets. The classical symbols were also used in alchemy for the seven metals known to the ancients, which were associated with the planets, and in calendars for the seven days of the week associated with the seven planets. The original symbols date to Greco-Roman astronomy; their modern forms developed in the 16th century, and additional symbols would be created later for newly discovered planets.