Now and forever. Amen.
Rules 5, 11, and 13, to refrain from the wanton giving of offence, to eschew unfairness, meanness and deceit, and at all times to speak the truth, of all the rules, these pertain most to the everyday domestic life of the knight.
Remember that the Knight is a new class within the Nobility during this time period, and the people who populated this class were initially military horsemen, lacking sophistication. And since the Nobility is a system of relationships, and therefore a system of trust between the parties to each relationship, the knights had to learn how to become trustworthy in all circumstances, particularly within the domestic sphere.
Now, remember also that the domestic sphere for the Noble class didn't just include one's immediate family. A Nobleman owned a manor and land, and this includes the employment and housing of all the people needed to operate such an estate. It would have included farmland, livestock, and craftsmen to support the manor, as well as servants within the manor, who tended to the everyday needs of the Lord and Lady and their family.
Noblemen needed to know how to interact with all of these people in such a manner as to keep the peace, to ensure their employees were happy and fulfilled, so that their estate would be prosperous. Essential to this would be honesty and thoughtfulness.
Deliberate OffenceNobody, no matter how careful he is, is able to refrain to causing offence to others. This is true for the simple fact that everyone is different, with their own sensibilities and value systems. This is recognized in this rule, and what is actually prescribed is to refrain from giving deliberate offence.
And think about it, when would anyone want to deliberately offend someone else except when that someone else is asking for it? Generally, people don't go around flipping people the bird for absolutely no reason. Either, there is a perceived or real offence already, or that someone else is being very difficult, perhaps deliberately so.
Actually, perhaps this isn't true. With a position of authority can sometimes come an air of superiority, and perhaps a sense of being "beyond reproach." From this, a bullying manner can derive, one of "looking down" on others as though they are less than you because of their difference in class. For example, it could become easy to be overly critical of others' work just because they are part of an "inferior" class, even if their work is excellent.
What this rule prescribes, then, is to refrain from social vengeance. It is to control oneself in the face of belligerence, ineptitude, or aggression. It is also to exercise humility and recognize that you need the people who work for you as much as they need you. It requires being thoughtful and considerate with other people, whatever class they belong to, and to refrain from being prideful in your Noble position.
Unfairness, Meanness, Deceit
These three are grouped together because they are interrelated. Whether speaking about business transactions, or the simple treatment of others, one should always deal fairly, kindly and honestly. And kindness really goes hand in hand with fairness, in the Christian worldview. Why? Because we believe that every person is made in the Image and Likeness of God, and is therefore owed the dignity and respect that such a nature bestows. So, to treat one fairly, according to the fact they are Children of God, one ought not to treat them with meanness or deceit.
And fairness is really a matter of justice. It is to treat each thing according to what it is, according to its nature, its reality. So, this wasn't just a matter of trade. It was a matter of, really, all social interaction. It is to recognize one's position when dealing with those of higher nobility. It is to discipline those in your employ equally according to the offence committed. It is to treat all with dignity and respect, and to do all of this honestly. It is NOT to behave spitefully.
And that's how deceit is tied together with these others. Whenever one deliberately treats another unfairly, one necessarily acts dishonestly. Why? Because being deliberately unfair implies you know what actually is fair, but then to act contrarily to what you know to be true. And often, treating someone unfairly means convincing them that they are getting fair treatment.
And all of this is to establish trust. Trust is the foundation of all relationships. Without it, relationships break down. And in trust, there is no room for dishonesty. Therefore, the Noble Knight must always speak the truth. The actual wording of this rule is even stronger: at all times to speak the truth. At all times, speak the truth.
In battle, your fellow soldiers needed to trust you, to be able to rely on you to watch their backs. In marriage, your wife needed to trust you, to be able to rely on the fact you would not betray your fidelity to her while away from the manor, that she was your sole desire. Your Liege Lord needed to be able to trust you, that you would remain loyal, true to your oath, that he could call on you in his time of need. Your employees, all who worked on your manor, needed to trust you, to know that you would not treat them unfairly, that you had their best interests at heart, and that they could rely on you to protect them when the manor was attacked.
A Nobleman was a man of great responsibility, who needed to meet the needs, not just of himself nor even just those of his family, but many, many people beyond that. They needed to know he was an honest and trustworthy man. Therefore, it was exceptionally important that he always spoke the truth. Otherwise, once caught in deception, the bonds of trust that he had to maintain would begin to corrode, and in time, he would lose everything. Especially on the battlefield, if his fellow knights didn't trust him, he would very quickly come to a gruesome end.