I'm a little curious what safeguards were there against people pretending to be someone else. I understand the system ran on honor, but there are always people willing to take advantage. I imagine messages could take a while to come through and maybe that would give you more opportunity to escape without anyone ever finding out your true identity, or something along those lines.
Basically, could that have happened, or is it a scenario that only has downsides?
Ransoming prisoners was a serious business (in a literal sense) that involved many parties on both sides, plus the courts if necessary (most of what follows is taken from Ambühl, 2013). The proper identification of the prisoner was important, notably to establish his value, which was based on his (alleged) wealth, on his reputation, and on other factors such his family and friends, his relation with the king, the existence of other prisoners in the opposite camp etc.
For the prisoner, there were no downsides - except in isolated cases - to have his nobility status and identity known to his "master", ie the person who had taken his oath to be good and loyal to him (the master was often the captor, but not always: a low-status captor, an archer for instance, could transfer his rights to a prisoner to someone of higher rank). Noblemen who surrendered expected to be spared and to enjoy as good conditions of imprisonment as their status allowed. They could envision their release by paying their ransom, by a prisoner exchange, by switching sides, or by escaping. This did not always work and some prisoners spent decades in prison (if the ransom was too high or if their master did not want to let them go), and some died. There were also cases where surrendering still resulted in death, as happened in after the Battle of Agincourt, when Henry V had many prisoners killed, even some who would have raised large ransoms.
If declaring oneself to be of noble rank was imperative, it could be dangerous to reveal one's identity immediately: circa 1420, Jean Courtois de Mons, aka Sicile, herald of Alfonso V of Aragon, wrote that a prisoner should avoid telling his name to his captor, for fear that the latter may know him and decide to kill him on the spur of the moment. Once that moment had passed, however, making known who you were was imperative. Ambühl cites one example of a high-ranking noble who tried to get away by hiding his status and identity, and it did not end well:
The importance of being identified, however, is sadly illustrated by the death of Anthony of Burgundy, duke of Brabant, on the field of Agincourt. According to chronicler Edmond de Dynter, the duke was captured and died anonymously in the slaughter of prisoners which took place towards the end of the battle. He had not been identified because he was wearing the armour of one of his chamberlains and declined to disclose his identity in the hope of paying a low ransom.
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