When two or more small amplitude waves simultaneously pass through a point, the disturbance at the point is given by the sum of the disturbance each wave would produce in absence of the other waves. To put this rule in a mathematical form, let `y_1(x, t)` and `y_2 (x, t)` be the displacements that any element of the string would experience if each wave travelled alone. The displacement `y(x, t)` of an element of the string when the waves overlap is then given by

`y (x, t) = y_1 (x, t) + y_2 (x, t)`

The principle of superposition can also be expressed by stating that overlapping waves algebraically add to produce a resultant wave. The principle implies that the overlapping waves do not in any way alter the travel of each other.

If we have two or more waves moving in the medium the resultant waveform is the sum of wave functions of individual waves.

`text(For example)`
Fig. a sequence of pictures showing two pulses travelling in opposite directions along a stretched string. When the two disturbance overlap they give a complicated pattern as shown in (b). In (c), they have passes each other and proceed unchanged.

An illustrative examples of this principle is phenomena of interference and reflection of waves.