Jointing in rocks is a key to the strains and movements they have undergone. In rocks which have had a simple history we expect to find a simple arrangement of joint planes: in old rocks we expect to find evidence of several series of tectonic movements and hence complicated jointing. For example, in the Cambrian quartzite south of Dublin the joint planes appear to run in all directions with no obviously dominant trend. But if say fifty readings taken haphazard in one of the quarries are carefully analysed it becomes evident that the joints tend to lie in certain directions, which, if a graphical method is used, appear as “peaks” on the diagram. On examining other exposures it becomes clear that some of these peaks are of regional significance, for they recur on the diagrams for many or all of the quartzite quarries. The diagrams also show, what is not evident in the field, that in certain directions jointing is almost entirely absent.