What do credit ratings like AAA, BBB, BB actually mean, and how do they affect bond yields?
I'm studying Fixed Income for CFA Level I and the credit rating system seems straightforward on the surface, but I want to understand how agencies assign ratings and what happens to yields across the rating spectrum. Also, what's the significance of the BBB/BB boundary?
Credit ratings assess the probability of default — the likelihood that an issuer will fail to make timely interest or principal payments. The three major agencies (Standard & Poor's, Moody's, and Fitch) use similar but slightly different scales.
Rating Scale Overview:
| S&P/Fitch | Moody's | Category | Meaning |
|---|---|---|---|
| AAA | Aaa | Investment Grade | Highest quality, minimal risk |
| AA | Aa | Investment Grade | Very high quality |
| A | A | Investment Grade | Upper medium quality |
| BBB | Baa | Investment Grade | Medium quality |
| BB | Ba | Speculative (Junk) | Speculative elements |
| B | B | Speculative | High default risk |
| CCC | Caa | Speculative | Very high default risk |
| D | C | Default | In default |
The BBB/BB boundary is critical — it separates investment grade from speculative grade ("junk" bonds). This matters because:
- Many institutional investors (pension funds, insurance companies) are restricted to investment-grade bonds only
- A downgrade from BBB to BB (a "fallen angel") triggers forced selling, causing sharp price drops
- Credit spreads widen significantly at this boundary
Yield Impact:
Higher risk = higher required yield. The extra yield over a risk-free government bond is the credit spread.
Rating agencies consider:
- Business risk (industry, competitive position, management)
- Financial risk (leverage, coverage ratios, cash flow)
- Country and regulatory environment
- Qualitative factors (governance, strategy)
Exam tip: Ratings are lagging indicators — markets often reprice bonds before the agency acts. Also remember that ratings address default risk, not interest rate risk or liquidity risk.
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