Both transmitting and receiving equipment operate on specific frequencies; the higher the construction quality of a set, the more accurate is the matching between the real operating frequency and the indication on the frequency dial. Highly accurate frequency stability can be achieved by using crystals. Since crystals are expensive, specific crystals have been provided for only a few frequencies in a set, or there have been ideas to find ways to achieve high frequency accuracy on many frequencies with a small number of crystals.
Frequencies are measured in Hz (oscillations / second), or in multiples of Hz, so kHz (1 kilohertz = 1000 Hz), MHz (1 megahertz = 1000,000 Hz) and GHz (1 gigahertz = 1000 MHz). In the early days of wireless telegraphy, frequencies were also indicated as wavelengths in metres; the formula 300'000/wavelength in metres can be used to calculate to the frequency in kHz.
In historical sets, frequencies above 30 MHz (in the VHF range used today by military forces) were sometimes calles ultra-short waves (UKW), which explains why the „UKW receivers“ of the German Wehrmacht, for example, operate on frequencies around 30 MHz and not as you would expect from the name in the FM broadcast band.