Growth onto vicinal substrates causes 60° misfit dislocations to adopt line directions away from <110> in order for them to maintain their presence within the substrate to strained layer interface. Observations show that for the growth of an on-axis [001]wafer the dislocations have a line direction, within measurement error, exactly [110] or [-110] and two sets of orthogonal dislocations are generated. When grown onto a wafer that is cut off-axis toward [010] four sets of dislocations are generated. The two sets of dislocations in each direction converge to form low angle intersections from which edge dislocations are formed. These edge dislocations can become very long by the glide out of the interface plane of the component 60° dislocations. This ‘zipping-up’ to form the edge components only occurs in one direction from the low angle point of intersection and the edge segments are exclusively generated in the buffer layer. Their density and penetration are a function of thickness and composition of the mismatched epilayer. The mechanisms by which the dislocations adopt line directions away from <110> and why they zip-up from the intersection in only one direction are discussed.