The Last Supper measures 450 × 870 centimeters (15 feet × 29 ft) and covers the back wall of the dining hall at Santa Maria delle Grazie in Milan, Italy. The theme was a traditional one for refectories, but Leonardo's interpretation gave it much greater realism and depth. The lunettes above the main painting, formed by the triple arched ceiling of the refectory, are painted with Sforza coats-of-arms. The opposite wall of the refectory is covered by the Crucifixion fresco by Giovanni Donato da Montorfano, to which Leonardo added figures of the Sforza family in tempera. (These figures have deteriorated in much the same way as has The Last Supper.) Leonardo began work on The Last Supper in 1495 and completed it in 1498—he did not work on the painting continuously. This beginning date is not certain, as "the archives of the convent have been destroyed and our meagre documents date from 1497 when the painting was nearly finished."
The Last Supper specifically portrays the reaction given by each apostle when Jesus said one of them would betray him. All twelve apostles have different reactions to the news, with various degrees of anger and shock.
In common with other depictions of The Last Supper from this period, Leonardo adopts the convention of seating the diners on one side of the table, so that none of them have their backs to the viewer. However, most previous depictions had typically excluded Judas by placing him alone on the opposite side of the table from the other eleven disciples and Jesus. Another technique commonly used was placing halos around all the disciples except Judas. Leonardo creates a more dramatic and realistic effect by having Judas lean back into shadow. He also creates a realistic and psychologically engaging means to explain why Judas takes the bread at the same time as Jesus, just after Jesus has predicted that this is what his betrayer will do. Jesus is shown saying this to Saints Thomas and James to his left, who react in horror as Jesus points with his left hand to a piece of bread before them. Distracted by the conversation between John and Peter, Judas reaches for a different piece of bread, as, unseen by him, Jesus too stretches out with his right hand towards it. (Matthew 26: 17-46). The angles and lighting draw attention to Jesus, whose head is located at the vanishing point for all perspective lines.
The painting contains several references to the number 3, which represents the Christian belief in the Holy Trinity. The Apostles are seated in groupings of three; there are three windows behind Jesus; and the shape of Jesus' figure resembles a triangle. There may have been other references that have since been lost to the painting's deterioration.