Duels had something akin to referees - seconds, as well as observers.

The job of the seconds was to negotiate fair and agreeable terms for how the duel would be conducted. They would select a time, a ground, approve the weapons, and so forth. They were selected for their own reputation and experience, so as to quell any suspicion of foul play in the setup. Additional observers might include surgeons (to deal with medical trauma), lawyers (who might have been involved in the dispute in other ways, if it was more than just a personal spat), and various rubberneckers who got wind of the affair and were spectating. Because of the nature of affairs of honour, many of these would be people of reputation, whose testimony would be taken very seriously by society at large, or the authorities, as the case may be.

At Aldo Nadi's duel with Adolfo Cotronei, Nadi reported the presence of

a small crowd of celebrated artists, famed writers and journalists, and great sportsmen. Also, several well-known fencing masters and amateurs. Among the masters you quickly detect the one who has trained your adversary...

So, all told, you could have quite a few witnesses to the affair, and since the ostensible purpose of the duel was to restore honour, the primary thing they were hoping to witness was the respectable behaviour of the antagonists in the settlement of the matter.

Nadi even claims the police are lurking nearby, apparently in no great rush to prevent the duel, but ready to prosecute if it goes badly:

The police always manage to know in advance where duels take place, but as a rule they are inclined to rush to the spot only when the whole thing is over.

There were a few ways it could all go sideways.

• The duellists could be insincere, colluding in a mock duel for the sake of social grandstanding. If the witnesses suspected insincere play, the muttering and rumouring that would result from the duel could quite severely damage both duellists' reputations, producing the opposite effect to what they intended. Unless, of course, the duellists were sincere, but it was agreed by the seconds that the dispute was silly, in which case they might themselves collude (by using wax or cork balls instead of lead, or weak amounts of powder, for instance) so that the duellists could trade an honourable number of shots with (supposedly unbeknownst to them) less-deadly ammunition. If they missed, nobody would be the wiser, perhaps not even the duellists!

• One of the duellists could balk or display cowardice, such as by refusing to take his position, or retreating beyond the bounds of play. That would terminate the affair, with a devastating effect on their reputation.

• One of the duellists could simply throw protocol to the wind and deliberately break the rules. With sufficient witnesses, this would not only destroy their personal reputation, but could easily result in prosecution, especially if injury or death resulted. Yes, murder would be a likely charge. Without witnesses it was a murky and sordid situation. Lord Byron vs. Mr. Chaworth is an instructive case, and was held up at the time as a prime example of why you ought never duel without reputable witnesses. Byron did in fact face prosecution, although it was in the House of Lords, and he was permitted to buy his way out of trouble.

• One of the duellists could commit a protocol error, such as by firing out of turn or striking late after seconds had intervened. This could be a grave mistake, but as long as the error didn't result in a serious wound or death, there was still a chance for seconds to patch things up and restore order, or perhaps rearrange the duel for another time. But if order could not be restored, that could lead to a whole new grievance, accusations of dishonour, and perhaps more duels if the seconds started feuding. If the error resulted in serious injury, you had a very awkward situation indeed, and a lot would depend on how the seconds assessed the situation, and what the witnesses saw.

In theory the legal system could take over at any time. Duelling was quite illegal in just about all times and places. You didn't even have to fight. The mere giving or accepting of a challenge was sufficient grounds to prosecute in most jurisdictions by the late 19th Century. But prosecutors then, as now, had discretion in which cases to pursue, and local tolerances for the tradition affected how quickly prosecutors chose to intervene. If, as in Nadi's case, they let it play out, then a lot rode on whether witnesses were left with an impression that justice had been served or corrupted by the extra-judicial affair.

Interestingly, in Nadi's case, he admits to feeling a murderous impulse after being embarrassed by a late touch from his opponent. Fortunately, reconciliation was forced upon him before he could do anything about it, and the affair was concluded to the satisfaction of everyone, even the police who showed up later. By that evening, everyone was dining together and toasting each other's fine character.