Source: U.S. Department of the Treasury  

Release: Corporate Bond Yield Curve  

Units:  Percent, Not Seasonally Adjusted

Frequency:  Monthly

The basic type of selected bond is analogous to conventional Treasury coupon issues: a bond that pays a fixed semiannual nominal coupon denominated in U.S. dollars until maturity, when the principal is returned. The HQM yield curve uses data from a set of high quality corporate bonds rated AAA, AA, or A that accurately represent the high quality corporate bond market.

For more information see https://www.treasury.gov/resource-center/economic-policy/corp-bond-yield/Pages/Corp-Yield-Bond-Curve-Papers.aspx