"The current theories dealing with the mechanics of joint development are discussed critically. An analysis of the stress conditions which can exist in competent rocks in the crust is presented. The conclusion is reached that 'master' and 'regional' joints can be, generically, either 'tensional' or 'shear' structures. They are, however, post-tectonic phenomena which develop as a result of uplift. Joints form a part of 'movement pictures' because their orientation is determined by residual stresses. Reasons are given which indicate that the mechanism described is generally applicable to joint development in folded sediments, igneous rocks, and also to incompetent material."