Our FM broadcast band extends from 87.9 to 107.9 MHz. 87.9 is not assigned, except to a few experimental stations. Is isn't assigned because of the proximity to TV channel 6's audio carrier, which falls at 87.75 MHz, and can be received on many FM radios. Other parts of the world have different FM bands. In Japan, for example, the FM band ranges from 76-92 MHz.
Some countries also space their stations differently.
For AM, stations are assigned by the FCC and are all 10 kHz apart. Our AM band runs from 540 to 1700 kHz. Some countries, including Canada, assign stations to 530 kHz. In the rest of the world, AM stations are spaced 9 kHz apart. Like frequencies, a station can request a specific call sign, if it is available.