How did The Am Law 100 make all that money? By maneuvering through a complex and churning market for big-firm legal services that in 2012 saw a group of core large clients buy fewer hours than they had in 2011 while being charged more for their purchases. Year-over-year demand fell by 3 percent even as invoiced rates for associates and partners increased by 2.6 percent.

Those are the top-line conclusions drawn from a fascinating data set from TyMetrix, the electronic billing giant. Working with The American Lawyer, TyMetrix gathered three years of billing records from its $42 billion LegalVIEW database to show the actual purchasing habits of large and midmarket corporate clients. In all, TyMetrix assembled $6.47 billion in Am Law 100 invoices submitted to 68 clients—27 are Fortune 500, another 10 are Fortune 1000 companies. Those clients are sprinkled across nine sectors, including banking, technology, industrials, and health care. One caveat: The invoices reflect what the firms charged, not what they collected. The invoices include up-front rate discounts, but this analysis does not capture final payments.