How we cite our quotes: (Chapter.Page)
Quote #1
As [Tristan] lay there, the Irish king's seneschal passed by and saw only the dead dragon. He hastily cut off its head and bore it triumphantly back to the palace, where he announced that he had slain the dragon. He thought that the dragon's real slayer must have been killed, and he claimed Yseut as his prize. (1.43)
The whole point of the Irish king's stipulation that the man who kills the dragon gets Yseut is to ensure that she ends up with a knight strong enough to protect her. The seneschal's deceit undermines all that. This episode introduces the idea of Yseut being given to the wrong man, something that will later haunt her relationship with both Mark and Tristan.
Quote #2
When it was time for Mark and Yseut to retire Tristan conducted them to their chamber and extinguished all the lights, explaining that this was an Irish custom. Brangain took Yseut's place in bed without Mark noticing the substitution; when the king was asleep Yseut replaced Brangain and the deception was successfully accomplished. (1.45)
Brangain has to take Yseut's place in the marriage bed because Yseut has already lost her virginity to Tristan and is afraid Mark will notice. Now there's a plan. Some people think Mark's inability to recognize the switch is evidence of his stupidity or that he does not really "deserve" Yseut. Of course, he has just met her, after all. Plus, it's dark. It's also a pretty common thing to happen in medieval literature, and even Shakespeare uses it in some of his plots. So make of it what you will.
Quote #3
'Tristan, for God's sake, it is very wrong of you to send for me at such a time!' Then she pretended to weep. For the sake of the Lord who created all things, never send for me again. I am sorry to say this, Tristan, but I am sure I should not dare to come. The king thinks that I have been wicked enough to love you. But before God I swear I have been loyal: may He scourge me if anyone has ever had my love except the man who had me as a maiden.' (2.48)
A common medieval anti-woman proverb said that "deceit, weeping, and spinning" were God's gifts to women. Well, it's 2 for 3 as far as Yseut is concerned. Here Yseut foreshadows the oath she will take in Part 15 when she swears before God that only Mark and the leper who carried her across the ford have been between her thighs. It's technically true that the only man Yseut has loved is the one who had her as a maiden. But that's not Mark; it's Tristan. The oath Yseut swears here is called an "equivocal oath." It's an oath that is true to the letter, but not the spirit, of the law. It plays on Mark's misconception about the identity of the man who took Yseut's virginity, relying upon deception for its effectiveness and validity.
Quote #4
'[Mark] is very angry with me because of his evil counsellors; he is very wrong to believe them, he does not realize how they have deceived him. (2.50)
Tristan and Yseut continually insist that the barons are deliberately deceiving Mark. The irony, of course, is that Tristan insists this while pulling Mark's leg and insisting that he and Yseut are not lovers.
Quote #5
'Tristan, queen, listen to me carefully: to escape the shame and to cover up the wrong we shall have to think of some suitable falsehoods.' (12.100)
The "suitable falsehoods" that Ogrin advises Tristan and Yseut to tell Mark are that they have never been lovers and that Tristan only fled with Yseut to protect her. The fact that it's a religious man advising them to engage in this deception might be the story's way of saying this behavior on the part the two lovers is okay. This also suggests that Tristan and Yseut always did right to attempt to hide their adultery from Mark.
Quote #6
'Where are you from, leper?' asked the king. 'From Caerleon, the son of a Welshman.' 'How many years have you been living away from people?' 'For three years, sire. I am not lying. For as long as I was healthy I had a very noble lover, now because of her I have these bumps on my skin.' (15.132)
Here, Tristan pretends to be a leper in order to help Yseut with the oath she is about to swear. Like the oath she swore to Mark earlier, Yseut's deception depends on Mark's misunderstanding of someone's identity in order to work. Notice that much like Yseut's equivocal oaths, a lot of what Tristan says to Mark here is true: he has been living away from people for about three years, and it is because of his noble lover—Yseut—that he has bumps on his skin, since it was she who prompted him to don his disguise.
Quote #7
'King Mark,' he said, 'whoever advised you to make this accusation did you a terrible wrong and certainly acted disloyally. You are easily influenced, but you must not believe false words.' (15.140)
Arthur is correct that Mark is easily influenced, but not to say that Mark has believed "false words." What his barons have told Mark is technically true. However, their words may be false in the sense that they are not uttered with Mark's best interests at heart, but out of jealousy and spite for Tristan.
Quote #8
'She has done everything that justice demands, and she put more into her oath than the villains required her to. She needs to make no defence concerning the king and his nephew beyond what all of us have already heard. She swore in her oath that no man ever came between her thighs, except the leper who carried her across the ford yesterday morning and King Mark, her husband. A curse on anyone who mistrusts her now!' (15.142)
The audience for Yseut's oath believes she has done more than was required by giving the details of her sexual history, since in fact all that she was asked to swear was that she never had Tristan as a lover. Yseut's oath here is "equivocal" because it is technically true, but misleading. It depends on everyone's misunderstanding of the identity of the leper who carried her across the ford, and on their assumption that he has been between her thighs only once.
Quote #9
Yseut of the White Hands had heard Tristan's instructions to his messenger, and out of jealousy she told him falsely that the sails were black. Believing that at the last his beloved Yseut had failed him, Tristan fell back on his bed and died. (19.165)
It's possible to read Tristan's death as a taste of his own medicine, in more ways than one. He has received his wound helping a friend carry on an adulterous affair. (What does this say about Tristan's attitude toward adultery in general?) But he dies because of a lie his wife tells. Given the lies and deception in which Tristan and Yseut have engaged, their deceit-driven death seems somehow appropriate.