A simple theoretical evaluation of the conditions conducive to maximum tightness of a tube joint is presented. The residual pressure between tube and plate, which can be considered as a measure of the tightness, possesses a maximum at certain tube dimensions. If tube and plate are of the same material, the tightness increases with increasing ratio of outside to inside diameter of the tube, up to a ratio of 1.4. If the tube is made from a metal sufficiently stronger than that of the plate, the residual pressure becomes larger, possessing a maximum at a ratio of 1.65 of the tube diameters.