Military call letters are the phonetic alphabet used in military communications when speaking on a radio or in other circumstances when clarity is crucial. Each letter is substituted by a full spoken word that starts with that letter.
The NATO phonetic alphabet, also known as the International Radiotelephony Spelling Alphabet, has been adopted by many English-speaking international organizations. The International Maritime Organization, the American Federal Aviation Administration and the International Telecommunication Union all use this method to communicate. Many similar phonetic alphabets existed and were commonly used by British and American military forces; however, the current NATO phonetic alphabet was developed in the 1950s and adopted by the International Maritime Organization in 1965.
The alphabet is intended to prevent miscommunication, which is likely when voice signals lose clarity over radio or because of other interference. To avoid confusion between similar-sounding letters such as "M" and "N," a person using the NATO phonetic alphabet would use the words "Mike" and "November" to distinguish between the two. For example, if a military officer needs to communicate "Ohio," he or she would say, "Oscar, hotel, India, Oscar."
For similar reasons, certain words are used in place of more common language in radio communications. For example, "yes," "no" and "help" are easily confused for other words; therefore, "affirmative," "negative" and "mayday," respectively, are used instead.