The airline industry began using teletypewriter technology in the early 1920s using radio stations located at 10 airfields in the United States. The US Post Office and other US government agencies used these radio stations for transmitting telegraph messages. It was during this period that the first federal teletypewriter system was introduced in the United States to allow weather and flight information to be exchanged between air traffic facilities. While the use of physical teletypes is almost extinct, the message formats and switching concepts remain similar.