For about a decade it has been noticed from the measurements of many jets that the value of the preferred mode and the spreading rate vary within a range of about 100%. In the present paper it is suggested that extremely low‐level spatiallycoherent disturbances in individualfacilities change the initial conditions of a laminar shear layer. The various initial conditions are able to cause different downstream developments of the jet and result in the discrepancies reported in the literature.

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