In Letter Sixteen, Cécile tells Sophie that the Chevalier Danceny has surreptitiously sent her a love letter. He attached the letter to her harp before putting it back in the closet after her music lesson and asked her to be sure to practice later. Cécile wonders whether she should consult the Marquise de Merteuil for advice.
Letter Seventeen is the confession of love in question. Danceny begs Cécile to tell him if she loves him or not. He can be saved from sadness by a single word from her ("un seul mot peut combler mon malheur"). He also notes that they can use the storage closet for further passionate correspondence.
Cécile writes to Sophie again (Letter Eighteen) in great confusion. Danceny showed up for her most recent music lesson more ardent than ever. She could hardly bear to look Danceny in his amorous eyes, but she promised to reply to his letter. Cécile then replies to Danceny (Letter Nineteen) with some uncertainty. She confesses that she is sorry that he is unhappy, even if she cannot do anything to make him feel better. He should feel consoled, at the very least, that there is no one else competing with him for her affections. And, more importantly, he should tell no one else what he feels.
Letter Twenty is a particularly wicked message from the Marquise de Merteuil to Valmont. She regrets her faithlessness to her chevalier, in her thoughts, at least, but she cannot help wishing that she and Valmont were together again. She sets out a single condition for their reunion: Valmont must sleep with the Présidente de Tourvel and obtain written proof of this tryst. Merteuil goes on to muse about Cécile's interactions with Danceny and the possibility that she has begun to rival Merteuil in his affections. The Marquise then concludes that perhaps the best plan is to make Cécile her student. She claims that this will be a kind service to Cécile's future husband, the Comte de Gercourt—it seems safe to assume that the Comte will be none too pleased to receive a young wife who is quite so experienced.
Insults and flattery play an important part in this particular series of letters. There is an interesting exchange between Valmont and Merteuil (letters ten, fifteen, and twenty), in which the reader can see their respective strategies at work. The Marquise begins the exchange by openly challenging Valmont, making him out to be a "slave," "a man who is down" ("un homme à terre") in his relations with the Présidente. She unwisely compliments herself on her own style of making love to the Chevalier—this will serve as material for Valmont's reply. Valmont further heaps compliments on her, and in doing so, raises her to such a height that she seems to be stooping too low with the inexperienced Danceny. Having now insulted Merteuil's choice in a backhanded way, Valmont compliments himself on his own resourcefulness against female wiles. In her reply, Merteuil calls Valmont out on his flattery, but his gentler words have drawn her out and his sudden interest in her activity tempts her. Thus Merteuil concludes in proposing the deal that will allow Valmont back into her bedroom, which is quite possibly exactly what he wants. Now he can continue to seduce Tourvel as far as he finds pleasing and also be assured of a warm welcome at the Marquise's when he returns to Paris. Love is portrayed here as a battlefield, and through language, the combatants seek to gain advantage over each other.
Indeed, whether the Chevalier Danceny's sad eyes and sad letters for Cécile are caused by a heart in love or parts farther south, which would like to remain in the Marquise's good graces, is another question. But we do not have anything to go on, in terms of what is really happening, other than the letters themselves. Thus, the question of sincerity keeps coming back in. It acts as a plot device, in fact, urging us to read on in hopes of finding some proof of one or another character's true emotions.