Yet already in the prophet Amos we read the rebuke: "You who lie upon beds of ivory" (Amos 6:4). In Homer, too, the ancient Greeks do not recline, but sit at table.
The custom of reclining at table was probably borrowed from the Far East. We do not know where it began, but we suppose it was borrowed from the Persians, though there are no direct indications of this.
The Roman custom assigned one couch to three persons. According to the Greek, or Athenian, custom, only two persons occupied one couch, and the head of one came to the height of the other's breast.
All reclined on the left elbow, so as to have the right hand free. Those reclining on one couch could speak with one another. One lay obliquely on the couch and could see over the head of the first reclining person the one who lay on another couch behind him.
But that latter person could not speak with his neighbor except by turning his head backward. See various articles and drawings on this in the dictionaries of Smith and Kitto, the Real-Encyclopedia, and others.
We have entered into these explanations so that one may understand why it was easier for Peter, lying on the second couch behind the Lord, to signal John. John was lying obliquely on the couch with the Lord. It was easier than addressing Jesus himself with a question.
Likewise, from this description of the table with its couches, it is clear that it was entirely convenient for John to bend forward only slightly, lean upon the Lord's breast, and whisperingly ask Him about the doubt troubling them all.
Likewise, the Lord's answer, falling into John's ear, remained completely unknown to the other disciples. This is clear from verse 28. After the Lord gave the piece of bread to Judas, none of them understood why the Lord had given it to him.
Nor did they understand why He said to him, "What you do, do quickly." Yet it is notable that John himself also seems not to have understood, or not fully understood, Judas's betrayal. As Chrysostom indicates, he did not dare think that a disciple could reach such great iniquity.
In general, the disciples' eyes were closed. As Chrysostom very rightly notes, the Lord deliberately did not reveal the betrayer clearly to them. It could have happened that, in the first impulse of their indignation, the betrayer would have been killed and torn apart by them on the spot.
Chrysostom also explains why Peter, while all the disciples look at one another in fear and amazement, signals John to ask the Lord and does not dare address Him with a question himself. Chrysostom says that Peter is still in some confusion after being rebuked concerning the washing of the feet.
We have seen above that the very arrangement of the table helped Peter give a silent sign to John. This was all the more so because he felt that the question could not be asked aloud. It was evident to all that the Lord did not wish to name the betrayer directly.
We should also note that all the smallest details recorded by the Evangelist certainly testify that the Gospel was written by an eyewitness of these events. The periphrasis by which Saint John always avoids his own name is deeply moving in its humility.