A harmonic is a wave whose frequency bears a whole number relationship to the frequency of a reference signal. Complex sounds, such as the human voice or the sound of a stringed instrument always consist of a fundamental frequency and then a number of harmonics, which, depending on their amplitude, will make up the basic characteristic of that sound. So the second harmonic is always two times the fundamental, and so on through the rest of the harmonic series. Discordant sounds have several or many harmonic waveforms that do not bear the whole integer relationship present in the basic sound.