The "Rules of Chivalry" varied somewhat from country. France and England shared much the same code because at the time of the HYW the nobility of both nations were still French speaking.
A modern incarnation of the Rules of Chivalry is the Boy Scout Oath. In general, however, you were supposed to play fair, protect and be nice to those of lower station than your own (lese oblige). Unfortunately chivalry was more often honored in the breach than the observance. Stature is based on how well you observe the rules of chivalry as unchivalrous actions will decrease your stature.
An example of national differences was the custom of German knights to throw their noble captives into a dungeon while French and English captives were wined and dined by their captors.
The Church was often considered a bastion of "goodie two shoes" who were overly sensitive to matters of Chivalry. Nevertheless, if the Pope stands for chivalry, crossing him can be unwise.
Chivalry also included attention to and respect for titles of nobility. Nobles were generally addressed by their highest rank. Thus Jean de Grailly, the Captal de Buch, would be addressed variously as, "My dear Captal," "Lord de Buch," "Buch," or "My Lord Captal." The family name was rarely used, except perhaps in legal proceedings. In other words, the title was more important than the family or given name. This practice continues to this day. The close friends of Charles, the Prince of Wales, will generally call him "Wales," rarely Charles and never Chuck or Charlie.
Once a year, the Kings of France and England as well as the Holy Roman Emperor (for players in the Empire) and the Pope (for players in Spain and Italy) may knight three NPCs. The NPC will have "Sir", "Chevalier", "Ritter" added to their first name and get a one time stature boost of "1". There is menu item to accomplish this. Only NPCs of exceptional merit should be so honored, not just your favorite bailiff.