The seating of guests in chairs, implied in Leonardo da Vinci's Last Supper painting, were not used for this most solemn occasion. Based on Jewish law and tradition, Passover (and most meals) was partaken of while people reclined around a low, long oval table. Each person would be lying on their left side and leaning on their left arm, with their feet behind them and their heads facing the table.
This traditional way of sitting on their left sides meant the disciple's bodies were somewhat facing the end of the table. This position, during a meal like the last supper, frees up the right arm for eating.
Jesus was the host of this unique "supper" (it was actually an observance of the Passover). Although the Bible does not directly state the seating for this last meeting, we can deduce, however, where Judas, Jesus, Peter, and John had to have sat.
In the above diagram of the last supper seating, Judas is sitting to the left of Jesus, in the place designated for the most honored guest. Although the host would normally select who sat next to him (see Luke 14:7 - 11), no record exists of Christ asking Judas to sit next to him. Judas likely felt he deserved to be the honored guest at the supper and quickly claimed the choice position as his right (Life and Times of Jesus the Messiah, Book 5, Chapter 10).
It should also be noted that Peter, in the seating diagram above, is located across from John at the end of the table. This was, especially during this most solemn "last supper," the lowest and most humble place at the table.
What evidence from the Bible exists to show that the above seating arrangement for Christ's last gathering with his disciples is correct? Scripture states that Peter had to get John's attention in order to request that he ask Christ who will betray him (John 13:21, 24). This would not be needed if Peter sat next to John. Peter, however, had to be close enough to John such that his request could NOT be heard by others. John then leans on Jesus' chest to ask him Peter's question (John 13:23 - 25).
Given how people sat to partake of a meal, John had to be immediately to Jesus' right while at the table. This position enabled him to slightly lean back and be against the Lord's chest. Additionally, the exchange that involved Peter, John, Christ and Judas was quiet and close enough such that the other disciples at the "last supper" had no idea what was said (see John 13:28). This meant they did not know the betrayer would be the one given a piece of bread (the "sop") during supper.
The host of a formal meal or supper would give the "honored guest" at the table the first "sop." The sop was a piece of bread or other small amount of choice food that is dipped in a sauce. The host, once he had the sop, would place it into the mouth of the honored guest. This guest always sat to the left of the host, whom the Bible clearly states was Judas Iscariot (John 13:21, 25 - 26).
After he received the sop, Judas asked if HE was the betrayer, which Christ affirmed that he was (Matthew 26:23 - 25). Satan then immediately possesses Judas and he leaves the room.
The nine disciples who did not hear what was discussed are clueless regarding what was happening (John 13:28 - 29). After Judas left and "the last supper" finished, Jesus did something that caught the disciples off guard. He took off some of his clothes, wrapped a towel around his waist, and then began at Peter to wash the disciples' feet (John 13:3 - 8)! Peter was the first to receive this ceremony since he sat at the end of the table. His seating also explains his initial impulsive rejection of having his feet washed (John 13:6).