What is the "tignon", and what is its importance?

The tignon is a particular style of headscarf worn by Calinda throughout the narrative, and which Cass, in emulation of her friend, puts on at one point (Chapter 6), only to have it ripped off by the angry Delphine. According to the author's afterword, the tignon became the accepted headgear for women benefitting from the plaçage system - women of the time had to cover their heads, the women of plaçage wearing the headscarves of female slaves decorated with feathers and jewels to proclaim their heightened status. Delphine's angry comment in Chapter 6 that Cass has not "earned" the tignon suggests that for Delphine, Cass has not reached a high enough financial or social status. It is, in other words, a negative judgment of the sort that, ironically enough, Delphine refuses to accept from other people about HER. She is, in this moment, a hypocrite.