Abstract A branch line is the line of intersection between two hard-linked fault planes, or between two parts of a single fault plane of more complex geometry. Of interest is whether they provide any information about the kinematic development of the fault system to which they belong. Analysis of branch lines from a variety of normal fault networks, interpreted on seismic reflection datasets, shows that the branch lines are generally aligned parallel to the extension direction. This relationship is shown to be a feature of polymodal (orthorhombic) fault systems produced by three-dimensional strain. Branch lines between bimodal faults (conjugate, with opposing dip) tend to be perpendicular to the slip direction.