A properly timed premature beat may be able to activate the normal myocardium but not the diseased myocardium. This figure illustrates the non-premature beat and an early premature beat which arises at the interface between the normal and diseased myocardium. Thed coupling interval of the premature beat is equal to the refractory period of the normal myocardium, and will be able to activate the normal myocardium (region A) because it will have regained its excitability. However, the premature impulse will not activate the diseased myocardium (region B} because it will still be refractory. For this reason, the impulse will propagate around the circuit in a counterclockwise direction. This is the concept of unidirectional block. The time required to traverse the circuit in this counterclockwise direction will allow the diseased region to recover it excitability and the impulse will now be able to conduct slowly in a retrograde direction through the diseased region to re-excite the normal region.