Off-The-Run Treasuries
All Treasury bonds and notes issued before the most recently issued bond or note of a particular maturity. These are the opposite of "on-the-run treasuries".
Investopedia Commentary
Once a new Treasury security of any maturity is issued, the previously issued security with the same maturity becomes the off-the-run bond or note.
Because off-the-run securities are less frequently traded, they typically are less expensive and therefore carry a slightly greater yield.
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See also: On-The-Run Treasuries, Treasury Bill, Treasury Bond, Treasury Note