This Article reports an empirical study of the network composed of patent “nodes” and citation “links” between them. It builds on an earlier study in which we argued that trends in the growth of the patent citation network provide evidence that the explosive growth in patenting in the late twentieth-century was due at least in part to the issuance of increasingly trivial patents. We defined a measure of patent stratification based on comparative probability of citation; an increase in this measure suggests that the United States Patent and Trademark Office is issuing patents of comparatively less technological significance. Provocatively, we found that stratification increased in the 1990s during the “patent explosion.” Here we report a further study indicating that the trend toward increasing stratification leveled off beginning around 2000. This observation suggests that there was a de facto tightening of patentability standards well before the doctrinal shifts reflected in the Supreme Court’s flurry of patent activity beginning around 2005. We also investigate the possibility that changes in our measure of stratification are due to something other than changes in patentability standards. While not conclusive, our results suggest that neither shifts in predominance of technological areas nor changes in citation practice account for our observations. We have thus identified an apparent puzzle: What happened around 2000 to cause a de facto tightening of patentability standards at the USPTO?