Two orientation relationships (ORs) and morphologies with well-defined facets of Mg2Sn precipitates in a Mg-Sn-Zn alloy were recently reported. However, the irrational angles of these two ORs were described without explanation. This study reveals that a unique matching-row-on-terrace structure exists in a dominant facet corresponding to either of the observed ORs. The calculated ORs, the orientations of three facets associated with each OR and the terrace-ledge structures in the dominant facets all match well with observations. The present method of row-matching analysis is useful for understanding high-index faceted interfaces and their associated irrational ORs.