A concave seat tube nut in AN3, used to terminate a rigid brake or clutch line where the mating component uses a concave seat. The nut captures the flared end of the tube and draws it against the seat in the fitting, creating a metal-to-metal seal with no washer or sealant involved. Seat style is critical here and the two common types are not interchangeable: a concave seat nut mated to an inverted flare port will weep no matter how much torque you apply, and on a brake circuit that is not a fault worth living with. Identify what your master cylinder, caliper or proportioning valve actually uses before ordering, then flare your line to suit.