Abstract The leading and trailing sunspots of two adjacent active regions are observed to collide as the evolving regions expand along the same direction. During approximately four days of this collinear motion, fibrils linking the colliding sunspots form a pattern suggestive of a potential magnetic field. No flares can be associated with the collision at this stage. Within a single day, and without an apparent change in the direction of the spots, the pattern of fibrils changes to a non-potential configuration. This onset of shear occurs rapidly in the absence of grazing motions. Thereafter, one spot splits, grazing motions develop, and shear is greatly enhanced along the line of polarity inversion. Sustained subflare activity begins after the onset of shear; stronger flares erupt as shear is enhanced. These circumstances are consistent with concepts based on shear as an essential ingredient of flares and which require critical levels of shear to be exceeded in order to trigger flares.