(adj.) describing a deal, bargain or exchange that clearly benefits one side more than the other
A Diomedean exchange is one that is clearly of greater benefit to one side more than the other. It’s a term used of all manner of deals, bargains, contracts and other business arrangements—not just straightforward swaps and exchanges—that appear similarly lopsided.
At the root of this expression is the name of the legendary Greek hero Diomedes. He was the son of Tydeus, one of the so-called Seven Against Thebes—a united group of warriors from Argos—who waged war on the city of Thebes in an attempt to restore Oedipus’ son Polynices to the throne. According to Homer’s Iliad, Diomedes apparently inherited his father’s bravery and valour, and during the Trojan Wars proved himself just as exceptional a warrior as the likes of Achilles and Ajax.
The expression Diomedean exchange relates to event in the Iliad, in which Diomedes was confronted on the battlefield by another valiant young hero, Glaucus of Lycia.
Glaucus was the grandson of Bellerophon, an ancient king of Corinth known for slaying the monstrous Chimera. On encountering Diomedes, the pair begin readying themselves to fight, and Glaucus begins boasting of his fighting prowess by announcing that, as the grandson of the great Bellerophon, he is always ready for battle at any given moment.
On hearing this, however, Diomedes immediately lays down his weapon. Turning to Glaucus, he explains that his own grandfather, Oeneus, was a good friend of Bellerophon—and so, despite being on opposing sides, he does not believe that he and Glaucus should fight.
Amazed by the unexpected connection between himself and his apparent enemy, Glaucus agrees and lays down his weapon too. Then, to cement their newfound alliance, Diomedes removes his armour and hands it over to Glaucus—who reciprocates, and the pair swap their armour right in the heat of battle, before going their separate ways with their friendship still in tact.
As touching a tale as this is, however, that wartime exchange was not quite all it seemed.
Diomedes’ armour, Homer explains, was made of leather and bronze, and only worth “nine oxen”. Glaucus’ armour, however, was solid gold and worth ten times as much. Symbolically swapping their armour was a clear display of honour and mutual respect; economically, it wasn’t the best exchange Glaucus could have hoped for. (In fact, the deal is so one sided that some versions of this tale have Zeus temporarily dulling Glaucus’ senses so that he doesn’t appreciate how ill-advised what he is doing truly is.)
It’s from this story that the expression Diomedean exchange ultimately developed—or, as it is also known, depending on what side of the bargain you’re on, a Glaucus swap.