A hub dedicated for fixed gear use has two threads the inner thread has a larger diameter. The fixed sprocket screws on here on a standard right-hand thread. The outer thread has a smaller diameter a lock-ring screws on here on a left-hand thread. A hub with only one thread can be used with a fixed sprocket but the wheel can’t be used as a foot brake. The sprocket may unscrew under the force of braking.
Unscrew The Lock-ring
Use a lock-ring spanner to unscrew the lock-ring by turning it clockwise.