Revisiting Credit Spreads & Quality Spreads
Corporate bond spreads, or the difference in yield between a corporate bond and an equal maturity Treasury bond, measures the relative risk of owning various corporate bonds across the risk spectrum relative to the "risk-free" rate.
As the yield spread between corporate bonds and Treasury bonds rises, this typically indicates increased levels of risk, worsening market or economic conditions or general health of corporations including debt levels and the ability to repay obligations.
I have covered the BBB corporate bond bubble in the past which brought in an analysis