The U.S. military uses the NATO phonetic alphabet to ensure clear radio communications. It consists of 26 words starting with the respective letters and the numbers zero through nine. When using letters or acronyms over radio communication, the phonetic alphabet is used.
The phonetic alphabet currently in use by the United States military is Alpha, Bravo, Charlie, Delta and Echo; then, Foxtrot, Golf, Hotel, India and Juliet; next, Kilo, Lima, Mike, November and Oscar; then, Papa, Quebec, Romeo, Sierra and Tango. Uniform, Victor, Whiskey, X-ray, Yankee and Zulu, as well as the numbers zero through nine are also used as part of this system. Zero is always referred to as zero, never "oh," and nine is pronounced "niner."
The military isn't the only organization that uses the phonetic alphabet. Police and airlines use it to ensure names and locations, such as grid coordinates, are clearly spoken. In the case of police and military, radio signals can be drowned out by gunfire and letters can sound too similar.
The current alphabet, as created by NATO and approved by the International Civil Aviation Organization, has been in use since the 1950s. Prior to the 1950s, several similar alphabets were in use. The present one was adopted because it uses words with sounds that persist across English, French and Spanish alphabets.