The best way to describe the relationship between the lords and vassals in the epic poem El Cid is that both are bound by reciprocal duty and obligations. The vassal owes his lord deference or respect for his authority and whatever service is due to him. Often this is a percentage of the peasant or yeoman's harvest for the lower or middle classes or military service for the upper classes. In return for fulfilling these duties, the vassal can expect the military and legal protection of his lord. In the case of the upper orders, a vassal's talents might be rewarded by the patronage of his lord, as in the case of El Cid. Of course, talent in arms was highly valued in a country under foreign occupation and constantly at war, so there were always ways for talented and energetic men of low birth to rise in service to king and country. A squire might become a knight, a knight a baron, a baron a count, a count a marquis and so on in a great chain that extends up to the king and above him, to God.

In exchange for deference, a lord owed his vassal condescension, which held virtually the opposite of today's meaning two hundred years ago. It was the reciprocal posture to the deference shown by an inferior to a superior in rank, and it required a lord to show kindness, decency, and respect to those of lower rank. For example, when a peasant by the roadside pulled off his hat and bowed to a lord riding by, the lord was required by the unwritten laws of courtesy to reciprocate by pulling off his hat to show respect in return. These mutual obligations bound everyone in a hierarchical society together. Every man and woman had their place, performed their duties, and showed proper deference to those above them and condescension to those below them in rank.

On the one hand, lords and vassals inhabit completely different social strata and have different standards for behavior and comportment. On the other, as the example of El Cid himself indicates, there is plenty of room for talented, upwardly-mobile people to join the ranks of the nobility. El Cid is the son of a commoner, yet, by the end of the story, he is not only powerful, but considered noble.

The poem also allows modern readers the opportunity to glimpse what it means to be noble in medieval Spain. Certainly birth has much to do with it, but what really seems to define nobility is the willingness to give away the wealth that one has accumulated through battle. El Cid, even though he has been exiled, gives a percentage of the booty he takes from his enemies to the king and he gives money to the Church.

There is some truth to the idea that El Cid is simply buying nobility through his generosity, but in a medieval context, this was precisely the expectation. Nobility and leadership was defined as the ability to win battles and take booty, which could then be distributed in a way that cultivated influence. This, ultimately, was what a lord owed to his vassals. The defining characteristic of a lord in El Cid's society was military prowess, not really tempered by generosity, but in fact military prowess with generosity as its end.