The title compd. is monoclinic, space group I2, with a 12.551(2), b 5.0137(10), c 12.700(2) .ANG., and b 104.62(2) Deg; Z = 4, dc = 2.017, dm = 1.987; R = 0.0339, Rw = 0.0341 for 2232 reflections. At. coordinates are given. The discussion contains comparisons with the structures of K H tartrate and Ca tartrate tetrahydrate. The title compd. belongs to the class of tartrates that are generally ideal for x-ray structure studies on single crystals within the framework of teaching seminars. The relevance of the powder x-ray diffraction method in the field of wine anal., with a view to detecting unallowed blending of wines, is emphasized.