This study of Old Spanish and present-day Mexico and New Mexico data develops a grammaticization account of variation in progressive constructions. Diachronic changes in cooccurrence patterns show that grammaticization involves reductive change driven by frequency increases. Formal reduction results in the emergence of auxilliary-plus-gerund sequences as fused units. Semantically, the constructions originate as spatial expressions; their grammaticization involves gradual loss of locative features of meaning. Semantic generalization among parallel evolutionary paths results in the competition...