This  article  describes  a  corpus-based  method  of studying  language  change ，focusing  on  the  recent  history  of British and American English grammar．  The method involves the comparison of matching corpora ，precisely and equivalently sam- pled from the use of the written language ，and separated by ( approximately) equal intervals of time． The study reveals strikingly significant patterns of  declining and  increasing  frequency :  for  example ，increasing  use  of  the  progressive  aspect  and  decreasing use of the passive voice．  As likely explanations for such changes ，processes such as colloquialization ，grammaticalization ，Ameri- canization ，densification and prescriptivism are invoked．