By studying the low-pressure Pena Negra migmatites, associated with the Avila batholith, in central Spain, we were able to identify what mineral reactions produced the different textural varieties of cordierite (metamorphic-, magmatic- and retrometamorphic-looking) commonly occurring in these rocks, and what role cordierite played in the low-pressure anatexis. Cordierite was not produced by the incongruent melting of biotite, but at subsolidus conditions by the continuous reaction: 0.96 biotite + 0.40 sillimanite + 0.33 quartz = 1.00 cordierite + 0.30 [higher-Ti and higher-Fe/(Fe+Mg)] biotite + 0.36 K-feldspar + 0.01 ilmenite + 0.03 water. When this reaction occurred in the presence of melt, normally zoned, magmatic-looking subidiomorphic crystals were formed. Large, inversely zoned, more magnesian crystals grew from the melt either on inherited crystals or from new nuclei. These processes happened sequentially as a result of advancing anatexis. Retrogression of garnet produced cordierite through the continuous reaction: 2.13 biotite + 1.38 garnet + 0.35 sillimanite + 0.09 quartz + 0.09 albite (from plagioclase) = 1.00 cordierite + 2.25 [lower-Ti and higher-Fe/(Fe+Mg)] biotite + 0.72 (higher-Mn and lower-Mg) garnet + 0.05 ilmenite + 0.001 anorthite (to plagioclase). At low pressures, the P-T path has a great influence on melt productivity. Within the temperature range of 660-750 degrees C, rocks that evolve along a pressure path low enough to intercept the invariant biotite + sillimanite + quartz = cordierite + K-feldspar + water at subsolidus conditions are more fertile than those evolving at higher pressure, although the chemical composition is the same. The presence of cordierite in source rocks increases their fertility in two ways: (1) directly, due to congruent melting at low temperature in the presence of quartz and K-feldspar, and (2) indirectly, because cordierite-producing reactions release water and increase the percentage of the haplogranitic component in the anatectic system through the release of K-feldspar.