Lekku, singular lek and also known as tchun-tchin or (by other species) headtails, were the tentacles hanging from the heads of all Twi'leks.
The head tails lekku protruded down the back of the skull and contained part of a Twi'lek's brain. The very word "Twi'lek" is believed to be derived from "twin lekku". A Twi'lek's head tails served many purposes in their daily lives and culture, storing fat and being used as erogenous zones. The Twi'lek's native language of Ryl was spoken using a combination of words and subtle movements of the lekku.
A Twi'lek's head tails were highly sensitive, and grabbing them forcefully was so painful that it could easily incapacitate almost any Twi'lek. Since they housed a Twi'lek's brain, damage to the head tails also caused lasting brain damage.
Long or multiple lekku were considered great status symbols, and often went hand in hand with respect, influence and wealth. A Twi'lek's head tails could be likened to phallic symbols, and for both sexes larger lekku brought with them some indistinct positive connotations. There were instances of Twi'leks born with triple or even quadruple lekku, and such individuals were common in the aristocracy of Ryloth, but many thought this was likely caused by inbreeding rather than genetic superiority.
Their head tails were a source of great pride to Twi'leks, especially when confronted by other species. The name "tchun-tchin" actually referred to each lekku—tchun being left leku while tchin was the right. In casual conversation the Twi'leks would usually refer to their lekku as 'tchun' or 'tchin.'