I have often heard Leonardo Da Vinci's famous painting criticized on the grounds that everyone is sitting all on one side of the table. The typical assumption is that he did this so the picture would look better due to not having to portray anyone with his back towards the viewer. It may be so that the picture is better that way, but the fallacy behind this assumption is the prior assumption that the Last Supper was conducted at a modern Western-style table. Actually, if you are going to criticize it on historical grounds, a more accurate criticism should be that they are sitting in chairs when the Bible indicates that they reclined, i.e. were lying down. Lying on one's side at the dinner table, though it may seem odd to us, is a typical ancient Greek/Middle Eastern custom. It is still practiced, I understand, today in the Middle East - I went to a Turkish tea house once where guests had the option to sit at Western-style tables or to recline Eastern-style.
In ancient Greek culture, including the Hellenized culture of New Testament Israel, one ate dinner reclining on a "triklinion" (or triclinium, if you prefer Latin), a set of three low couches placed either around three sides of a low rectangular table or around one side of a broad horseshoe-shaped table - the setup was designed with the idea that that way servants would be best able to bring dishes and wait on people with a minimum of inconvenience to the guests. St. John indicates that our Lord sat in the place of the host (ha! He's the host!), with the Twelve to His right and to His left. So regardless of the precise shape of the table, Jesus and the Twelve were all basically facing the same direction.

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