Abstract: Conjugate en-echelon vein arrays well-developed in the limestone of the Helvetic nappes of western Switzerland have been studied. They can be divided into two types: the primary and the surperimposed. The geometry of the veins and arrays have been described in detail. It may be concluded that the en-echelon veins discussed are synte-ctonic extension veins which were associated with the second order stress field due to the motion of the shear zone. The angle between the veins and array containing them and the conjugate angle between two shear zones have been measured. It is suggested that the former,varying from 20皌o 45?,depends on the normal stress acting on the shear zone. It is considered that the en-echelon veins were formed under the transitional condition from brittle fracture to ductile flow of the rocks. The generation of veins has been connected with the developmental history of the nappes.