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This diagram is a compilation of oxygen isotopic analytical data for meteoritic and lunar samples, collected since about 1973 in the University of Chicago lab of Robert N. Clayton and Toshiko Mayeda. ¹⁸O and ¹⁷O give the deviations, in parts per 1000 (permil; 0/00), of the ratios ¹⁸O/¹⁶O and ¹⁷O/¹⁶O, respectively, in samples relative to Standard Mean Ocean Water (SMOW). The diagram is, in effect, an isotopic map of solar system bodies. All terrestrial and lunar samples plot on the so-called terrestrial fractionation line (TF), which has a slope of ~1/2, along which the variation can be explained by simple mass-dependent, physicochemical processes such as evaporation, condensation, and igneous crystallization. Meteoritic samples mostly do not plot on this line, and either plot on separate slope-1/2 lines parallel to but displaced from TF, or else along linear arrays having slopes closer to 1. The Carbonaceous Chondrite Anhydrous Mineral (CCAM) line is the dominant slope-1 array, whose extreme ¹⁶O-rich end is defined by calcium-, aluminum-rich inclusions (CAIs). The CCAM line requires non-mass-dependent isotopic effects or mixing of an additional ¹⁶O-rich component. Some CAIs disperse to the right of the CCAM line owing to melt volatilization that resulted in massdependent isotopic fractionation (e.g., the FUN line).