As the use of airmail started gaining popularity there was a need to alert postal authorities that the mail was to be sent by air. Hence, mails were super scribed with the words “by air mail”. Soon air mails labels or etiquettes as they are called were issued, first by postal authorities and soon by airlines, hotels etc as these etiquettes did not have any value. Airmail etiquettes were soon issued in attractive colors and in a variety of shapes. Each country issued labels in their own language but the works “air mail” or “par avion” were mentioned on all etiquettes. Basic colors of blue, red & white somehow became the standard for all airmail etiquettes. Many countries cancelled or postmarked the label, perhaps that these should not be reused in the mistaken belief that etiquettes should be treated in same way as stamps.
Etiquettes issued by various airlines:
Airmail cover from Vienna to Bombay via Karachi. The airmail etiquette has been canceled at Karachi on 17th October, 1931.
Cover is postmarked at Vienna on 7.X.31 which must be the date of posting, but has actually been dispatched by airmail on 11.X.31 as seen by the circular postmark "WIEN 11.X.31 Flugpost"