Loyalty in a Knightly context is often reinforce by very formal oath, exchanged between Lord and Vassal. Knights swear to their King and Queen to be obedient and virtuour, by a variety of oaths, in some Kingdoms made up on the spot, in others every Knight of that Realm has sworn the same oath, from the very first to the most recent, and they all have that oath committed to their memory, a concrete guideline to follow as the engage in their daily lives.
Loyalty is not exclusively swearing to uphold a particular set of rules, though having those rules is a helpful guide. It is more the result of those oaths. Loyalty is the Virtue of being true to those to whom one is Sworn. Loyalty in the SCA context os expressed in terms of Fealty, being bound in a bilateral relationship of mutual defense and service. The Vassal swears to follow and enforce the legal commands of their Liege, join their army and champion their causes; the Liege swears to defend the Vassal from any offense, raising that same army to defend or avenge the Vassal when their lands are attacked.
One can swear to defend their King and Queen, and then insult them behind their backs. One can swear the same, yet stand by silently while others slander their Liege Lord and/or Lady, or one can display their Loyalty by not standing idle while others speak insultingly, but challenging those with mouths full of insults to speak with respect, even when the insults are true.
This leads to an interesting point. Whether Loyalty is the result of formal oaths, or just the bonds that form between friends, when one is disloyal, by breaking a public oath, or by betrayal of friendship, that is when Loyalty is tested, and this is when the other Virtues must rally forth. Sometimes it is right, by circumstance, to remain Loyal even though the other has not been, while in others, their disloyalty may be so great as to break all bindings of oath and allow liberty of action. How one thinks on the other virtues is an important guideline to determine the correct course.
Loyalty is a difficult Virtue on which to write, for the consequences are great regardless of the decision. One can be "Loyal to a fault", one can be entirely self-serving, having no Loyalty to anyone. Extremes should generally be avoided, and Loyalty measured by Justice, practiced with great Prowess and Courage, and always weighed by Franchise.