47 CFR §2.101

1. (a)The radio spectrum shall be subdivided into nine frequency bands, which shall be designated by progressive whole numbers in accordance with the following table. As the unit of frequency is the hertz (Hz), frequencies shall be expressed:
   1. (1)In kilohertz (kHz), up to and including 3 000 kHz;
   2. (2)In megahertz (MHz), above 3 MHz, up to and including 3 000 MHz;
   3. (3)In gigahertz (GHz), above 3 GHz, up to and including 3 000 GHz.
2. (b)However, where adherence to these provisions would introduce serious difficulties, for example in connection with the notification and registration of frequencies, the lists of frequencies and related matters, reasonable departures may be made. 1
3. (c)In communications between administrations and the ITU, no names, symbols or abbreviations should be used for the various frequency bands other than those specified in this section.