Did the concept of chivalry extend to commoners or anyone who wasn't considered nobility? If not, did knights generally avoid commoners? I apologize if I'm being too vague.
I tried to treat this subject in relation to another question (Were there any laws of war). That answer became a horrible convoluted mess and I was forced to abandon it before (ironically) writing the section that pertains directly to this. Thus I will keep this as short and concise as possible.
No. Chivalry was not a concept which was either aimed at or directed towards the 'commoners' /u/idjet is currently spoiling for debate on that massive can of worms here. The ideas that we conflate with chivalry (protecting women, protecting the poor, the church) emerged from a series of councils organised by the clergy in Occitania in the late- tenth and eleventh-centuries. These were called the Peace of God movement and were directed against the proto-chivalric class and mostly at the proto-nobility (ie. landlords) who were often violent in the practice of their lordship and private warfare.
Chivalry was a later creation, which I discuss in the post mentioned above, but to condense: it was the emergence of a conscious code which combined a warrior ethic with a Biblical one c.1170-c.1220. This conscious code was fluid, inconsistent, and occasionally thoroughly contradictory. But it was a shared discourse across several distinct polities but between individuals who could be loosely equated. It was this social class to whom the benefits of chivalry were aimed.
One example is ransom:
Another attribute (loyalty) meant not solely that one should be loyal to your lord but that one should be loyal to your word. Ransom was built on mutual respect and trust as it was likely the prisoner would be released merely for a pledge (ie. promise) to pay his ransom. This was a wider part of reciprocal obligations that the martial class imposed upon themselves and would rely on one another to enforce (not that they always did, of course). There were other factors at play. The Frankish diaspora and marriage among the nobility had created a situation where male kinsmen may well face one another while serving their lord or king. It also made sense not to kill those whose kith (friends) and kin could feasibly retaliate. The rise and popularity of the tournament in the twelfth-century created not only a point where large numbers of similarly-minded individuals could test their prowess and win honour, but could test these principles and loyalty to their vows. Finally, and made most obvious by the tournament, was the economic incentive. Ransoms were far more valuable than dead nobles and knights (in most cases).
There was little or no such incentive to preserve the life of those who could not pay a ransom. There are literally dozens of famous examples of the execution of common soldiers after a battle. The modern concept of the merciful knight has intruded from the historiographical conflation of the Peace of God movement and the Romanticisation of chivalry in the eighteenth- and nineteenth-centuries. Mercy was extended usually to your peers and the rest may as well go hang.
For a fascinating and in depth discussion of mercy and chivalry in France see:
For which the preface and the introduction are both available online.
To expand on this question, how did commoners treat knights?
Were they treated like heroes, hated, or something else entirely?