Symbolism, Imagery, Allegory
The first time Julien sees Mathilde de La Mole dressed completely in black, he has no clue who she could be mourning… since the rest of her family isn't wearing black. Is she channeling Lydia Deets? Nope: it turns out she's commemorating the day one of her long-dead ancestors had his head cut off. Oh, that's uplifting.
According to legend, the guy's lover took his head in her lap and kissed it. The story makes Julien realize that Mathilde has a poetic (read: gruesome) soul beneath all her shallow comments. As he thinks at one point,
At least he was loved, as perhaps it's sweet to be loved. What woman alive today wouldn't be horrified to touch her lover's chopped-off head? (2.10.33)
Frankly, Julien is impressed by Mathilde's ability to not be grossed out by the idea of kissing a severed head. Little does he know that she'll make good on this commitment later, because the book ends with her kissing Julien's own head after it's been sliced off. Yum.
As the novel draws to a close, we learn that "No one knew it but, alone in her covered carriage, she carried on her knees the head of the man she had so loved" (2.45.46). Love can make people do some pretty crazy things, and apparently, kissing severed heads is one of them.