Minecraft wiki entry for Logic, associated text: However, that is only true if the inputs are the same power level (or at least not different by more than 1), otherwise one signal could overwhelm the other's attempt to suppress its signal. If this circuit is sure to receive inputs of the same power level (because the system it's part of has been designed that way), then the "basic" version can be used. Otherwise, some method should be used to ensure the inputs are equal — for example, with repeaters (the "repeated" version) or with torches (the "inverted" version).