Terrestrial igneous rocks commonly record crystallization at oxygen fugacities more oxidizing than the WM (wüstite-magnetite) buffer and more reduced than a log unit or so above the nickel-nickel oxide (NiNiO) buffer. Their oxidizing conditions thus are not far from those of the FMQ (fayalite-magnetite-quartz) redox buffer. Nonetheless, there are systematic differences that correlate with tectonic setting. Igneous rock emplaced and erupted in island arcs typically record oxygen fugacities 1 or more log units more oxidizing than those of the NiNiO buffer. In contrast, basalt and gabbro in non-arc settings typically record oxygen fugacities from about those of the FMQ buffer to a log unit or so more reducing than that buffer.