In a J-K flip-flop, if both J and K are tied to same input, then the derived Flip-flop becomes T Flip-Flop (shown below ).
This has only one input which is called T-input.
|Input Tn||Output Qn+1|
When T is 1 this flip-flop becomes a toggle flip-flop, and the output Q keeps toggling on every clock edge(-ve). T Flip-flops are used in synchronous counter circuits.