From: 'St. Peter's - Guide to Basilica and Square'
The left transept resembles a chapel, open to all, but reserved for recollection and prayer. On weekdays, the basilica's scheduled Masses are celebrated at the main altar, dedicated to St. Joseph, at: 10, 11 and 12. On Sundays, only one Mass is celebrated at 13:00. Communion is also distributed here to the faithful who ask for it. The Blessed Sacrament, a memorial of Christ's love for us until the end of time, is constantly preserved in the Tabernacle. All the faithful who enter the basilica pass here and genuflect before the Blessed Sacrament, pausing a few moments, kneeling or sitting in the pews, to pray.

There are three altars. The Central Altar was previously dedicated to the Holy Apostles, Simon and Jude, and Thaddeus, whose relics are preserved in an ancient sarcophagus. John XXIII dedicated it to St. Joseph, Mary's husband and the putative father of Jesus. The mosaic which now decorates the altar was designed by A. Funi (1890-1972), and shows St. Joseph. Flanking the altar are two oval mosaic portraits of the two Apostles, after paintings by Vincent Camuccini.

One the right is the Altar of St. Thomas above which is a mosaic altarpiece inspired by a painting of the Disbelief of the Apostle Thomas, by Vincent Camuccini (1771-1844). The relics of Pope St. Boniface IV who consecrated the Pantheon for Christian worship are kept beneath the altar.

On the left is the Altar of the Crucifixion of St. Peter, with a mosaic reproduction of a picture by Guido Reni (1575-1642) showing St. Peter crucified upside down. An ancient tradition claims that St. Peter suffered martyrdom precisely where the altar stands. In fact part of the basilica is actually built on the site of Nero's Circus.

From: 'St. Peter's Basilica - A Virtual Tour' by Our Sunday Visitor
Entering now the left transept, one of the arms of the Greek cross, once known as the Tribune of St. Simon and St. Jude Apostles, due to the two saints' relics, whose remains, transferred from the St. Agnes mausoleum dating from Constantine's time, lie under the high altar. On the vault of this southern Tribune, as in the one on the opposite northern side, there are valuable stucco and gold decorations by Luigi Vanvitelli, with three representations in stucco, taken from Raphael's tapestries and illustrating the "Miraculous Catch," the "Healing of the Cripple" and the "Punishment of Ananias and Sapphira."

Also to be admired is the very elegant crafting of the capitals crowning the columns at the sides of the altars, in fluted porphyry, yellow and ancient yellow granite. The last of these flank the central altarpiece while two identical ones flank the corresponding altar dedicated to St. Processo and St. Martiniano in the opposite right hand transept.

From: 'THE NEW SAINT PETER'S'
The third arm of Michelangelo's Greek cross is focused on the south tribune, called that of Saints Simon and Jude, and contains, besides the immense vault decorated by Vanvitelli and Maini, three altars decorated with rare marbles.