FM radio stations in the U.S. transmit in bands between 88.0 megahertz (MHz) and 108.0 MHz. Inside this band, each station occupies a 0.1 MHz (100 kilohertz (kHz)) section. Skipping over the even-numbered radio stations (88.2, 88.4, 88.6, etc.) was done by the FCC to prevent radio stations from interfering with one another. Radio stations that are only 0.1 MHz (100 kHz) apart tend to bleed into each other more often.
The numbers on your radio dial represent frequencies used by your local radio stations. Megahertz means “millions of cycles per second,” so “91.5 megahertz” means that the transmitter at the radio station is oscillating at a frequency of 91,500,000 cycles per second.
In the vast majority of countries, FM (Frequency Modulation) radio stations use frequencies between the range 87.5 to 108 megahertz (MHz). To tune to a medium or channel, it will only work if the number ends with a decimal (even or odd). The decimal serves to divide the frequency band and establish safe ranges in which the signal will work well, that is, spaces that are free of interference. This has caused trouble for Americans in the US Armed Forces. Many radios intended for use in the US cannot tune to the even decimals – which results in poor reception of some of the AFN radio stations.
Because the lowest channel is centered on 87.9 MHz, the tenths digit (in MHz) of the center frequency of any FM station in the United States is always an odd number. Radio stations in the United States typically end in odd numbers, with a few exceptions. The practice of using odd numbers dates back to the early days of commercial radio when stations were assigned call letters by the government. In the early 1920s, the Commerce Department began issuing call letters to radio stations in an effort to regulate and organize the airwaves. Stations east of the Mississippi River start with "W," and those west start with "K."
The FM band from 88 to 108 MHz is arbitrary, based on history and convenience. The FCC decided on 200 kilohertz spacing on odd-number boundaries. This differs in Europe but works well to prevent interference between stations.
In Europe, FM stations are 100 kilohertz apart and can be even or odd numbers. AM stations can end in even or odd numbers too. For example, the Capital FM station in London is at 95.8 MHz, and BBC Radio 5 is at 909 and 693 AM.