  The examples of rhythmical signals with variable period are considered. The
definition of periodic function with the variable period is given as a model of
such signals. The examples of such functions are given and their variable
periods are written in the explicit form. The system of trigonometric functions
with the variable period is considered and its orthogonality is proved. The
generalized system of trigonometric functions with the variable period is also
suggested; some conditions of its existence are considered.
