Filling yarn insertion on single nozzle air jet looms depends on factors related to the nozzle, the air guiding system, and the yarn storage and feeding system. Weft insertion force depends on the pneumatic flow characteristics of the main nozzle. An introduction to the mechanics of air jet weft insertion addresses the design of the main nozzle, the velocity distribution and design of the air guide system, factors influencing weft insertion, effect of insertion speed, and filling yarn tension. Experiments showed that air velocity and turbulence at the nozzle exit decreased as the air tube length increased. The ratio of air guide diameter to nozzle diameter affected flow ragte in that larger ratios resulted in higher flows processed by the jet. The yarn tube must be positioned to create suction inside the yarn tube. 14 refs.