This article addresses the diachronic development of so-called rival forms, i.e., words or grammatical constructions that appear to be synonyms, based on a detailed empirical analysis of two seemingly synonymous constructions in Russian. Corresponding to the English ‘decade construction’ in the twenties, Russian has two rival constructions, viz. v dvadcatye gody [lit. “in the twentieth years”] (with the numeral and noun in the accusative) and v dvadcatyx godax (with the numeral and noun in the locative case). Three hypotheses about rival forms are considered: leveling (whereby one form ousts its rival), sociolinguistic differentiation (whereby the two rivals survive in different varieties of a language) and semantic differentiation (whereby the two rivals develop different meanings over time). Contrary to what has been suggested in the literature, the article found little evidence for semantic and sociolinguistic differentiation. Instead, it demonstrates that leveling is taking place, since the accusative construction is in the process of ousting its rival. While this study shows that corpus data facilitate detailed analysis of the interaction between leveling, sociolinguistic differentiation and semantic differentiation, the analysis also points to limitations, especially when it comes to corpus-based analysis of sociolinguistic and semantic factors.