Dendrite coherency (or impingement) is important for the formation of solidification structures. The effects of silicon concentration and modification of eutectic silicon crystals on dendrite coherency in Al-Si alloys have been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendritic coherency point in the alloys decreases with increasing silicon concentration, and varies from 0.09 to 0.39 when the silicon concentration in the alloy is in the range 0.5 to 11 mass%. Modification of eutectic silicon crystals may increase the coherency fraction solid somewhat, but not much. When a grain refiner and a modifier are added together the coherency fraction solid will increase greatly. A theoretical approach is proposed to describe how these factors affect the dendrite coherency in the alloy. (orig.)