U.S. Treasury Secretary Tim Geithner (L) shakes hands with China's State Councilor Dai Bingguo after their closing statements during the U.S.-China Strategic and Economic Dialogue at the Interior Department in Washington in this May 10, 2011 file photo. A little noticed 2009 change in how Washington sells its debt sheds new light on America's delicate balancing act with its biggest creditor, China. When the U.S. Treasury Department revamped its rules for participating in government bond auctions two years ago, officials said they were simply modernizing outdated procedures. The real reason for the change, a Reuters investigation has found, was more serious: The Treasury had concluded that China was buying much more in U.S. government debt than was being disclosed, potentially in violation of auction rules, and it wanted to bring those purchases into the open - all without ruffling feathers in Beijing. To match Exclusive USA-CHINA/TREASURIES REUTERS/Jason Reed/Files (UNITED STATES - Tags: POLITICS BUSINESS)