This altarpiece, known from its subject as the Madonna delle Nevi (the Madonna of the Snows) was originally commissioned for one of the oldest and most venerable altars in the Siena cathedral. It stood immediately left of the doorway known as the Porta del Perdono - the side entrance to the cathedral that provided most immediate access to both the baptistery and to the civic centre of the Palazzo Pubblico and the Campo.Sassetta received the commission in 1430 from Ludovica Bertini, the widow of Turino di Matteo, the man responsible for both the cathedral sacristy and the baptismal font. According to one local chronicler, Turino had died in 1423 and been buried in front of the Porta del Perdono. In the contract for the altarpiece, Ludovica makes it clear that she is commissioning the work both in memory of her husband and also in her own right as a pious Franciscan tertiary, so the coat-of-arms of her own family as well as that of her husband appear prominently displayed on the richly ornamented fabric covering the Virgin's throne. Her commitment to the Franciscan Order is clearly demonstrated by the inclusion of Saint Francis in the right foreground of the main panel of the altarpiece. The imagery chosen for the rest of the altarpiece, however, was entirely Sienese and civic in intention. It depicts the familiar subject of the enthroned Virgin with the Christ Child on her lap and surrounded by angels and saints. The altarpiece therefore echoes the imagery of two of Siena's most revered civic icons - the front face of Duccio's high altarpiece for the cathedraland Simone Martini's mural in the council hall of the Palazzo Pubblico. That such an association was explicitly intended is shown by the inscription engraved on the Virgin's halo: 'If you trust me, Siena, you will be full of favour'.The imagery of the altarpiece was elaborated in order to honour two of the Virgin's titles - 'Queen of Heaven' and 'Our Lady of the Snows'. Two angels behind the throne hold a crown over the Virgin's head. The angel on the left of the throne, meanwhile, carries a dish filled with snow and the angel on the right makes a snowball. The seven narrative scenes of the predella describe in detail the legend of Our Lady of the Snows. They show how, in the reign of Pope Liberius (352-66), the Virgin caused snow to fall miraculously in the heat of August on the Esquiline Hill in Rome. Furthermore, the snow fell precisely in the pattern of the ground plan of a church. The Virgin then instructed a wealthy layman and his wife, and Pope Liberius, to build a church in her honour on this site - a church that became Santa Maria Maggiore, one of Rome's major basilicas.