A single Block with fully functional lines-selection , which can switch one input signal to the two different output, suitable for the line selection mode of musical instruments、 amplifier、Sound box (need horn line), and the combination effect pedal. The selection-switch design can change the A/B separate transmission to be A&B simultaneous transmission. In both modes, ABY can be used in both directions, and the signal can be transmitted from interface A and B to interface Y, and from interface Y to interface A and B.