The classical geometric tolerance factor t = (r_A + r_X) / sqrt(2)(r_B + r_X) proposed by Goldschmidt (1926) for predicting perovskite stability. Values near 1.0 favor cubic perovskite; t < 0.8 or t > 1.0 favor non-perovskite structures. Achieves ~74% accuracy, superseded by Bartel's revised tau.