Abstract: A musical instrument tuning aid includes a stable source of reference signals. Those signals are divided by a selected amount to produce a given tone signal. A note produced by an instrument under test or measurement is detected and used to develop a corresponding sensed signal. Variations in phase between successive time increments of the sensed signal desirably are averaged so as to produce a measurement signal. The sensed or measurement signal is compared in frequency with the tone signal for the purpose of producing an indication signal. In response to that comparison, a display indicates whether the instrument is sharp, flat, or on-tune. A signal presence detector may be included for the purpose of disabling the display in the absence of a signal to be tested. Also preferably included is an arrangement for causing the on-tune display to flicker at a rate representative of the nearness to an exact match.