Was a time when there was none. A cniht was just a man who fought. (Indeed, originally he was just an attendant or servant, but what on earth did the king have men for except to fight for him?) That he was of noble birth stemmed from the nobles' keeping the fighting powers in their own hands. More or less, though they were known to suppress peasant militias, even ones that fought the Vikings and otherwise the king's enemies. This is why the Latin translation of "knight" is miles.
The rise of a rich merchant class made them feel the pinch, which is where they invented the ceremony of knighting. Fatal in some ways. Once you no longer have to fight to qualify as a knight, the fat rich merchant you minded so much can get himself knighted with a good fat purse, and there's the end of your distinction. On the other hand, the ceremony can lend a useful mystical aura to the role; Chaucer has a notable reverence for it.
And supposing you have knights and men-at-arms in the novel, there's the question of how to distinguish. . . .
In some lands, the knights will all be men of noble, or at least gentle, blood. Haughty about their prerogatives. Only the lowest ranks would even consider marrying the richest merchant's daughter, unless he had wangled some post in the royal court to get himself technically ennobled.
Other lands have a stout burgher and freeholder militia and would laugh to scorn the notion of a fancy ceremony.
The problem lands in the borderlands, where I have a fortress standing between the places of danger and the settled and more-or-less peaceful lands. The muse ambled along with men-at-arms and knights, but didn't go into detail. I suspect that a peasant who distinguished himself and desired it could get himself knighted there, but -- differences, differences -- the mystical aura could come in handy. . . perhaps the knights are oath-bound to the fortress, never to leave. Or perhaps they are the only ones bound to set forth into the places of danger at need. . . ponder, ponder, ponder. . .