Many shortwave receivers in the sixties and seventies had frequency dials with a poor accuracy and only coarse frequency marks.
In order to calibrate the dials, crystal calibrators were constructed. In these, a crystal oscillator generated provided harmonics usually on multiples of 1000 or 100 kHz. This signal causes a deflection on the signal strength instrument, and even better can be tuned to „zero beat“ in A1 (CW) operation mode: the receiver is carefully tuned until the whistle becomes lower and lower in frequency and disappears.
For reception, the crystal calibrator must of course be switched off, especially if you wand to tune the set to a frequency close to a calibration mark.
The possibility to achieve a good frequency dial accuracy by means of a crystal calibrator can be found especially in sets of the sixties and seventies. In some cases, an interpolation dial was provided, so that a frequency 10, 25 or 50 kHz higher or lower could be accurately tuned between two calibration points.
In some cases, external crystal calibrators were sold as accessories or constructed by radio amateurs themselves.