The Piccolomini Altarpiece is an architectural and sculptural altarpiece in the left-nave of Siena Cathedral, commissioned by cardinal Francesco Todeschini Piccolomini who expected it to become his tomb. However, he was elected Pope Pius III and buried in the Vatican. It was built between 1481 and 1485 by Andrea Bregno in Carrara marble, with additions in the following decades – these included four niche sculptures produced between 1501 and 1504 by Michelangelo of saints Peter, Augustine (later resculpted as Saint Pius), Paul and Gregory.

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