Introduction to Airmail etiquette / label

An airmail etiquette, often shortened to just etiquette, is an adhesive label used to indicate that a letter is to be sent by airmail.

Because the etiquettes are basically just instructions to postal clerks, and have no monetary value, their printing and distribution need not be as carefully controlled as for postage stamps, and most are privately produced. The usual design is a plain blue oblong, with the phrases such as "AIR MAIL" and/or "PAR AVION" (French, traditionally the international mail language) in white letters. 

However, at various times, airlines and hotels have produced more elaborate designs, some quite attractive.

The airmail etiquette may be omitted if airmail stamps are used on the letter, and in some cases even this is not necessary if a country sends out all its foreign mail by air. In some countries, such as, the United Kingdom you may simply write "PAR AVION -- BY AIR MAIL" on the envelope, even though etiquettes are available free from post offices.

An early catalog of etiquettes was published in 1947 by Frank Muller; more recently the Postal Label Study Group has put out the Mair Airmail Label Catalog, a 627-page work illustrating 3,289 types of etiquettes.

Source: Wikipedia