§ 39-17-1301

"Knife" means any bladed hand instrument that is capable of inflicting serious bodily
injury or death by cutting or stabbing a person with the instrument.

What is the length allowed under Tennessee law? 
Concealed or open, the length is 4 inches.  This is for a fixed or folder knife. 

An automatic knife or one that can be opend by inertia are illegal in Tennessee.