It's based on frequencies of letter combinations. Like 'E' and 'R' are together.

There is no absolute limit on frequencies. The frequencies can be almost arbitrarily small or big.

There is no absolute limit on frequencies. The frequencies can be almost arbitrarily small or big.

There is no absolute limit on frequencies. The frequencies can be almost arbitrarily small or big.

There is no absolute limit on frequencies. The frequencies can be almost arbitrarily small or big.

no

The tone frequencies, are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies.

Airband, a group of frequencies in the VHF radio spectrum, uses the frequencies between 108 and 137 megahertz. A license is required to operate airband equipment.