WITHIN the last year or so we have learned to glue metals together with a strength which brings this method of joining materials into competition with riveting, at least in the thin gauges used in the aircraft and motor industries. Apart from this new extension of gluing to the metal working trades synthetic adhesives have already revolutionized the woodworking industries. This revolution is due to the superior quality of the resulting products and the increased rate of output made possible by the intrinsic high speed of setting of synthetic adhesives aided by such novel methods as high frequency heating, infra‐red heating and the like. In aircraft in particular the “weather‐resistance” of synthetic adhesives has largely removed the disadvantages of wood construction, due to the use of casein glues, so much in evidence in the first winter of this war. It may be said therefore that gluing has been raised from the status of a useful but humble convenience of daily life to a process of engineering significance. But before engineers can use gluing in the fabrication of structures they must be provided with data sufficient to enable thorn to compute the strength of the joints, and we at Aero Research Ltd. have therefore endeavoured to find a simple relation between the strength of a lap joint and its geometry. Such a simple relation is found in “the joint factor” which is defined3 as the square root of the thickness of the sheet divided by the length of the overlap.