Treasury bills, commonly known as "T-bills," are short-term securities issued by the U.S. Treasury on a regular basis to refinance earlier T-bill issues reaching maturity, and to help finance federal government deficits. Of all money market instruments, T-bills have the largest total dollar value outstanding--a sum that as of 2004 exceeded $650 billion.
In addition to scheduling regular sales of T-Bills, the Treasury also sells instruments called cash management bills on an irregular basis, by re-opening the sales of bills that mature on the same date as an outstanding issue of bills.
When T-bills were initially conceived, they were given three-month maturities exclusively; but bills with six-month and one-year maturities were subsequently added. Three-month and six-month bills sell in the regular weekly auctions, and another bill auction takes place every four weeks for the sale of one-year bills.