The air index of a yarn supplies information on the behaviour of a yarn during insertion on an air-jet loom. The air index, obtained using the air index tester from Picanol, indicates the speed of the inserted yarn in relation to the yarn count. This enables the ‘air friendliness’ of different yarns to be compared. In addition to the yarn count, uniformity, elongation, etc., the air index can be used as a new criterion for characterizing the processing efficiency. Details are given of the operating principles of Picanol’s air index tester, and of how to calculate the air index.