By the use of suitable delayed feedback (d.f.b.) from the output terminals of a four- (or two-) pole network to the input, a great variety of different signals can be generated, the shape of which is determined by the characteristics of the four- (or two-) pole network, the d.f.b. loop and, under certain conditions, by the shape and duration of the trigger signal which is needed to set the d.f.b. system in operation. Two simple applications of delayed feedback in electronic circuits (where the loop gain is real and equal to or less than unity) are given:The output of a phase-inverting amplifier (amplification, -g) is fed by a delay line (delay time τ) to the input of the amplifier. If |g|ν < 1 (where ν is the loss in the d.f.b. loop), periodic signals of frequency f approximately = 1/2τ are generated.If, in the same arrangement as (a), |g|ν < 1, an output signal is produced only if a trigger signal is fed into the d.f.b. loop; that is, the circuit works as a triggered oscillator of frequency f approximately = 1/2τ. Because of mixing (additive or multiplicative) between the trigger signal of duration T and the d.f.b. signal, the shape of the output signal depends on the value T/τ.By investigation of the output signal with a frequency meter or oscilloscope, the delay time of an unknown element (e.g., delay line amplifier) connected in the feedback loop can be measured. The d.f.b. oscillators have a frequency stability of about 10^-5/h.