NATO Phonetic alphabet
Some letters may sound over a radio transmission very similar. To reduce the risk of a misunderstanding, all letters aren't spelled individually, but with a easy to differentiate, pre-determined word.
This also applies to numbers in a similar fashion:
0: "ZEE-RO" - 1: "WUN" - 2: "TOO" 3: "TREE" 4: "FOW-ER" 5: "FIFE" 6: "SIX" 7: "SEV-EN" 8: "AIT" 9: "NIN-ER"
Link to the offical NATO Page
