Cappella di San Pio V is a 16th century papal chapel in the Apostolic Palace of Vatican City.
The chapel is one of a series of private chapels in the Vatican Palace, used in turn as this pope or that moved the Papal Apartments (the suite where the pope actually sleeps) from one part of the palace to another.
So, it is a predecessor of the present Cappella Privata del Papa.
Pope St Pius V ordered the construction of the four-storey L-shaped range now in the south-west corner of the Cortile di Belvedere and facing the Cappella Sistina across the Cortile della Sentinella. The arrangements included a stack of three chapels, one above the other. The extant chapel, with its altar dedicated to St Peter Martyr, was the one in the middle, on the first floor. The pope had a small suite of rooms next to it.
The dedication was because the pope was a Dominican friar, as was the martyred saint.
However, the chapel was liturgically redundant because the Papal Apartments were in the so-called Palazzo di Sisto V. The status of pope's private chapel had already been lost to the Cappella di Urbano VIII in the 17th century, anyway, when the actual Apartments moved up one storey from the so-called Borgia Apartments to the Raphael Rooms.
So, Pope Pius IX gave the premises over to be part of the Museo Sacro (also known as the Museo Cristiano). In 1905 Pope St Pius X authorised the display an investigation of the reliquary at the Sancta Sanctorum of the Lateran (see San Lorenzo in Palatio ad Sancta Sanctorum). Some of these were laid out for display in a cabinet in the chapel in 1936.
The last restoration was under Pope St Paul VI (1963-78)
The chapel is a rectangular room, with a semi-circular sanctuary apse of the same width having a conch. Unusually, at the back of the apse is a rectangular niche framing a window with shutters -this is above the altar, where you would expect the altarpiece. The apse is separated from the main barrel vault by a triumphal arch.
A right-hand side door (which leads into what was the first room of Pope St Pius's apartments) is matched by a deep niche in the left hand side wall. This contains a display case, itself containing various treasures from the Sancta Sanctorum.
The chapel interior is entirely covered with fresco panels set in ornate stucco decoration, coloured in cream with gold highlights. Much of the decoration is grotesquerie, including flower swags with fruit, and the architectural forms hint at early Baroque. The frescoes are in a rather saccharine Mannerist style, in pastel shades -much of it might be restoration work of the earlier 19th century.
The "counterfaçade" wall (there is, of course, no façade) has a pair of large fresco panels showing SS Peter and Paul flanking the entrance. Above the latter is a panel displaying the heraldry of Pope Gregory XVI, which is topped by a broken segmental pediment with two pairs of curlicues at the break (a proto-Baroque detail). Above, in the lunette created by the barrel vault, are allegories of Faith (left), Hope (right) and Charity (middle); Faith is holding a yoke, and Charity has a horde of children which tells you immediately that this is not Our Lady.
The side walls have two large panels depicting more allegorical Virtues, before the side door on the right and the display cabinet on the left. The former has the Keys of Peter with cherubs over it, the latter has another pair of cherubs with the heraldry of Pope St Paul VI.
Pride of place in the display case is given to a cross reliquary with polychrome enamelled decoration depicting scenes from the Life of Christ, which dates from the reign of Pope Paschal I (817-24). It allegedly contained fragments of the True Cross, although when examined in the early 20th century it was filled with balsamic resin.
The barrel vault of the nave has a rectangular central compartment, itself containing a circular tondo surrounded by four pentagonal panels. The tondo displays an allegory of Truth Triumphing Over Falsehood. The young woman representing Truth is depicting unlocking the Book of Seven Seals, and trampling on two prone figures portraying Judaism and Islam (with a turban). The grotesquely prehensile second toe of her sandalled foot is typical of the Mannerist style.
The four surrounding compartments depicts four Dominican teacher saints: SS Dominic (with lilies), Albert the Great (the bishop), Thomas Aquinas (writing) and Vincent Ferrer (with an hourglass -his favourite theme was the Second Coming of Christ).
To the sides are six panels depicting more Virtues, the central pair being rectangular and the others oval.
The triumphal arch has a pair of shallow Tuscan Doric plasters, heavily ornamented with a pair of strap corbels near the tops. The archivolt is also ornate, with little frescoes of allegorical Virtues and a tablet commemorating the restoration of Pope Pius VII in the keystone.
The altar stands in front of a niche containing a shuttered window. One would expect an altarpiece depicting The Martyrdom of St Peter Martyr. The apse walls either side have a pair of large frescoes depicting two events in his life. The window niche is not flanked by any pilasters to create an aedicule, but over the conch entablature is a pair of nested broken pediments, the inner segmental and the outer triangular, both with curlicues on their broken ends.
The altar itself has a frontal in polychrome pietra dura work.
The semi-dome of the apse conch has three main fresco panels which depict three angels holding symbols of martyrdom -crowns and palm branches. The central one is a vertical oval intruding into the pediment below, whereas the side ones are trapezoidal. These main panels are separated and flanked by eight smaller ones in two horizontal registers, the upper ones having putti with musical instruments and the lower ones four popes. Each pope has a book, but is not further identifiable. Near the apex of the conch are three little panels with winged putto's heads.
The chapel is part of the full visitor's circuit of the Vatican Museums, and you pass through it after visiting the Cappella Sistina.