official library of the Vatican, especially notable as one of the world's richest manuscript depositories. The library is the direct heir of the first library of the Roman pontiffs. Very little is known of this library up to the 13th century, but it appears to have remained only a modest collection of works until Pope Nicholas V (1447–55) greatly enlarged it with his purchase of the remnants of the imperial library of Constantinople (now Istanbul), which had recently been conquered by the Ottoman Turks. Popes Sixtus IV (1471–84) and Julius II (1503–13) further enlarged the library, and under Sixtus V (1585–90) the architect Domenico Fontana (Fontana, Domenico) erected the library's present building. In the early 21st century the library possessed some 75,000 manuscripts (mostly in Latin or Greek) and more than one million printed volumes.