A signal, the shape of which when displayed is, well, square. In other words, it holds to a negative value for a time equal to half the wave's period, then switches abruptly to a positive value which it holds for the same length of time, then switches back negative again, and so on. How quickly it switches between these states is determined by how far into the higher frequencies the equipment in use can extend. Square waves rarely form part of music, and are primarily used as diagnostic tools, since inspection of how well equipment handles a square wave can reveal a lot about its high frequency handling and whether it shifts phase across its frequency handling range. For those who are mathematically inclined, a square wave can be defined by the formula, where f is the fundamental frequency and n is an odd natural number: sin(f) + 1/3 sin(3f) + 1/5 sin(5f) + ... 1/n sin(nf).