Consider what happens when banks sell their loans to Fannie or Freddie. A bank might write a mortgage at 5.1% and sell it to Fannie, which guarantees the loan and sells it with other loans packaged as mortgage-backed securities, perhaps with a coupon of 4.35%. The difference of 0.75 of a percentage point is booked by the bank, which uses some of that revenue to cover costs in its mortgage business. From 2000 through 2008, that margin averaged 0.73 of a percentage point, according to data from Barclays Capital. But in 2009, the average was a much wider 0.98 of a percentage point. Any additional margin likely boosted banks’ bottom lines. And by a lot, potentially, given that $1.4 trillion of mortgages were written in the first three quarters of 2009, according to Inside Mortgage Finance. Indeed, Wells Fargo and Bank of America, which together account for 45% of the market, reported blowout mortgage earnings last year.