In the late 1970s, the "wild card option" was often used to settle futures 
In the late 1970s, the "wild card option" was often used to settle futures contracts of these entities, which also uses the difference between the implied and actual repo rates drive the notion of "cheapest to deliver" for settling its futures contracts. Pension fund demand resulted in re-issuance of longer-dated ones in 2006, while TIPS are those which are tied to CPI. Regularly scheduled auctions replace the current "on-the-run" versions and supply the market with these semi-annually coupon-bearing entities, and the simultaneous purchase of long-dated and sale of shorter-dated ones in 2011 has been dubbed "Operation Twist." Generally regarded as risk-free assets, for 10 points, name these financial instruments that represent the debt of the United States government.
In the late 1970s, the "wild card option" was often used to settle futures contracts of these entities, which also uses the difference between the implied and actual repo rates drive the notion of "cheapest to deliver" for settling its futures contracts.
Treasuries [or Treasury securities; or Treasury notes; or Treasury bonds; or Treasury bill; or anything that mentions the word Treasury; prompt on "bond" or "note bill"; accept US bonds or US government bonds before mentioned]