Suppose there are two sources of waves of a given type. For example, there may be two loudspeakers emitting sound waves or two accelerating charges emitting EM waves. What is the resultant wave? Or, suppose that two pulses are sent down a string, one after the other, so that the second pulse “collides” with the first one after the first one has been reflected from the opposite end. What happens as a result of this collision? Such questions are answered by the principle of superposition, which states that: