Anyone who, by luck or effort, managed to obtain the training and equipment to be a knight, could eventually enter that class. In the course of the 12th century, a social and ethical dimension is added to this professional aspect. The strong influence of Cluny monks, who try to give an ethos to savage warfare, leads to the definition of the true miles Christi , a soldier who follows a certain code of behavior, which we now call chivalric. Thus, knights were not necessarily nobles, nor were nobles necessarily knights. The noble class and the knightly class slowly came to merge from the late 12th century onward.
Nobles become knights with increasing frequency.
Conversely, heredity enters the knightly class in the 13th century. This marked both the closure of the knightly class as well as the beginnings of a new form of access, by purchase. See also my page on women knights . As a military institution, knighthood was on the wane from the late 13th century on. The end of feudal society meant that sovereigns gained a monopoly on war-making, and the old form of military service owed to one's immediate lord became obsolete.