When a taut string is plucked, the resulting waves are non-linear. These non-linear waves are produced by the interference of two waves traveling in opposite directions that have the same frequency and amplitude. The resulting wave patterns, consisting of nodes and antinodes, are called normal modes or harmonics. The first harmonic occurs when the wavelength is twice the length of the string, which is also termed the fundamental mode. The boundary condition is that there are nodes at the two ends of the string. A wave that does not follow this condition is impossible. For waves that follow this condition, twice the length of the string equals the integral multiples of the wavelength. Based on this, the wavelength of each normal mode can be determined. Recall that wavelength is inversely related to frequency. So, the frequency of each harmonic is the nth multiple of the first harmonic.