58 It is not clear where Simon Peter was seated. If he were on Jesus’ other side, it is difficult to see why he would not have asked the question himself. It would also have been difficult to beckon to the beloved disciple, on Jesus’ right, from such a position. So apparently Peter was seated somewhere else. It is entirely possible that Judas was seated to Jesus’ left. Matt 26:25 seems to indicate that Jesus could speak to him without being overheard by the rest of the group. Judas is evidently in a position where Jesus can hand him the morsel of food (13:26).
59 Grk “to this one”; the referent (the beloved disciple) has been specified in the translation for clarity.
60 Grk “him”; the referent (Jesus) has been specified in the translation for clarity.
61 That is, who would betray him (v. 21).

In other versions

so Simon Peter motioned to him to ask Jesus of whom he was speaking.

Simon Peter motioned to this disciple and said, “Ask him which one he means.”

Simon Peter motioned to him to find out who it was he was talking about.

Simon Peter motioned to him to ask, “Who’s he talking about?”

So Simon Peter gestured for this one to inquire who it was about whom he was speaking.

Simon Peter therefore beckoned to him, that he should ask who it should be of whom he spake.

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