Fixed Income — Corporate

Core Concepts

Credit Spreads

Compensation for default risk, liquidity risk, and downgrade risk above the risk-free rate. Multiple spread measures exist with increasing precision:

G-spread (Government Spread): Bond yield minus interpolated Treasury yield of the same maturity. Simple but assumes a flat term structure between benchmark maturities.

Z-spread (Zero-Volatility Spread): The constant spread added to each point on the risk-free spot rate curve such that the sum of discounted cash flows equals the bond's market price. Superior to G-spread because it accounts for the full shape of the term structure.

OAS (Option-Adjusted Spread): For bonds with embedded options, OAS = Z-spread minus the value of the embedded option. OAS represents the "true" credit compensation after removing the option component. Requires an interest rate model to compute.

Credit Ratings

AAA/AA/A/BBB are investment grade. BB/B/CCC/CC/C/D are high yield (speculative grade). The BBB/BB boundary is the most consequential threshold — many institutional mandates prohibit sub-investment-grade holdings. A downgrade across this boundary ("fallen angel") forces selling by constrained investors.

Migration Matrix

A transition matrix shows the probability of moving from one rating to another over a 1-year horizon. A BBB-rated issuer has roughly 85-90% probability of remaining BBB, 4-5% chance of upgrade, 4-5% chance of downgrade, and a small probability (~0.2%) of default. Migration matrices are published annually by rating agencies.