A comparison of Old Spanish and present-day Spanish data provides evidence that reductive change in grammaticizing forms may be manifested not only as a diachronic process but also as a synchronic difference between formal and informal registers. Clitic climbing frequencies in Spanish auxiliary + gerund sequences have increased diachronically as part of a series of reductive changes and in tandem with construction frequency increases. In calculating construction frequency, both the token frequency of auxiliary + gerund sequences and their frequency relative to lone-standing gerunds turn out to be important. Differences between present-day conversational and written data show that clitic position is stylistically stratified. Register differences in clitic climbing are found to be linked to construction frequency as well, suggesting that frequency effects operate on a more general synchronic level as well as on the level of particular texts. These text-level frequency effects may be related to parallel structure “birds of a feather” effects.