Abstract Many folds of both deformation phases on Söröy exhibit a prominent lineation brought out by elongate micas or spindleform-quartz grains. This lineation displays a variety of patterns, depending on ils orientation relative to the fold axis. This dependence confirms the primary nature of the axial noncy lindrism. The lineation is a product of layer-parallel extension and represents the long axis of the sectional strain ellipse ( X 1), for the particular attitude of the layering. The formation of the lineation is confined to those layer orientations which cut the extension field of the strain ellipsoid. If the noncylindrical fold axes curve from normal into parallelism with the lineation, the patterns of lineation orientation developed will reveal the attitude of the major extension axis ( X) and indicate the nature of the strain. The lineation developed in the earlier stages of fold growth and became a passive marker in later fold modification. The regional pattern of the lineation is similar to that for minor fold axes, and together they are held to identify the XY-plane of the bulk finite strain-field.