Based on electron backscatter diffraction, a novel and simple method was developed to determine orientation relationships (ORs) between phases in solids. The method was successful in explaining the crystallography of heterogeneous nucleation of Mg grains grown on Al 2 Y nucleation particles in an Mg–10wt.% Y alloy. Two reproducible ORs were identified and they completely agree with previous predictions from the edge-to-edge matching model. This confirms that Al 2 Y is an effective grain refiner for this type of alloy.