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A Federal Reserve regulation that established the capital requirements for the banks.
Under this, banks were restricted from paying interest on demand deposit accounts. It also put a cap on the interest the banks could pay savings and NOW accounts.
This was essentially removed by the Dodd-Frank bill which became law in 2010. Now banks can offer interest, although they are not required, on demand deposit accounts.
The remaining aspects of Regulation Q which still exist are now a part of Regulation D.