Trial by duel was a judicial instrument in the middle ages and some time after and was used to determine whether the accused was guilty. This was done by a duel between the accuser and the accused. If the accuser won, the late accused had been guilty. Trial by duel was normally only used for murder cases or other capital crimes. But what was the reason for this absurd legal practice?
It was used because the courts often lacked the evidence needed to come to a sensible judgement. In the middle ages there was no way to record anything faithfully - no tape recorders, no film, no photograps. They also lacked fingerprint and genetic analysis, and had little understanding of criminalistic method. So they had to rely on eyewitness accounts (if there were any) and circumstantial evidence.
How then determine the guilty in a case of murder? By asking God. Trial by duel was a way of trying to force His hand. The accuser would battle the accused to the death. The idea was that God would not let the innocent perish and the guilty survive. In practice, trial by duel meant that whoever was stronger, won. God does not like it when people try to blackmail Him. The church was well aware of this and always opposed the trial by duel, with little success.
A somewhat similar practice was used by the inquisition. This often resulted in the impossible situation where an accused witch could either drown in the trial by water, or be burned at the stake.
The trial by duel at least gave one a fighting chance of survival. It must also be noted that though the duel was usually between men, there were cases where women participated as well. And they didn't always lose, either.