Treasury Yield
  
What's the yield of a T-Bond or T-Note or Treasury Bill? Whatever it is, it's low. Why? Treasury anything is backed by the U.S. Government. Safe. Liquid. Lots of buyers.
The U.S. Treasury sells some debt instruments at a discount to par; the return from that discount then defines the paper's yield. Other pieces of debt paper are structured more like vanilla bonds: they carry an interest rate and come due so many years later.
Big paper, big debts, big uncle. (Sam.)