Investment Grade

The rating scale runs from AAA/Aaa at the top through AA, A and BBB before crossing into speculative territory at BB/Ba. Within each band, agencies attach notches: plus and minus for S&P and Fitch, numerals 1, 2 and 3 for Moody's. The dividing line is therefore a single notch wide, and an issuer at BBB– sits one downgrade away from a change in status that has consequences far beyond the marginal deterioration in credit quality it reflects.

Ratings are supplemented by outlooks, typically running twelve to twenty-four months, and by credit watch or review designations that signal a near-term resolution. Agencies also distinguish the issuer credit rating from the rating on an individual obligation. Secured, senior unsecured and subordinated instruments of the same borrower are notched up or down from the issuer rating to reflect differences in expected recovery, so a company rated investment grade can have subordinated or hybrid paper rated below the threshold.

Much of the economic force of the designation comes from rules written around it rather than from credit quality itself. Broad corporate bond indices admit only issues meeting a minimum rating, so a downgrade triggers index exclusion at the next rebalancing and mechanical selling by passive and benchmark-constrained portfolios. Insurance regulators assign lower capital charges to higher-rated holdings, bank capital frameworks apply risk weights that step up sharply below the threshold, and many pension and mandate documents prohibit sub-investment-grade exposure outright.

Where agencies disagree, index and mandate rules apply a convention rather than a judgement: commonly the lower of two ratings, or the middle rating where three exist. A split-rated issuer straddling the boundary can therefore remain eligible under one framework and be excluded under another, which explains why issuers manage the marginal notch aggressively through asset sales, dividend cuts and equity issuance.

For investment-grade portfolios, realised losses from default are rarely the dominant source of underperformance; credit migration is. A one-notch downgrade repricess the bond even when no payment is missed, and the loss is approximately the change in spread multiplied by the spread duration of the position. Excess return over duration-matched government bonds can be decomposed into carry from the spread earned, capital gain or loss from spread movement, and any loss from default net of recovery.

Expected loss itself is the probability of default multiplied by loss given default, where loss given default equals one minus the recovery rate. Across the investment-grade band, historical annual default frequencies are very low and cluster near zero for the higher rating categories, so the observed spread substantially exceeds actuarial expected loss. The residual compensates holders for liquidity, downgrade risk and the systematic tendency of defaults and spread widening to arrive together in recessions.

Be able to place any rating symbol on the correct side of the boundary, translate between the two agency conventions, and explain how notching produces issue ratings that differ from the issuer rating, including structural subordination at holding companies whose claims rank behind operating-subsidiary creditors.

Be able to read a credit migration matrix, identify the probability of moving from one rating to another over a horizon, and estimate the price impact of a given transition using spread duration. Be able to compute and interpret the standard spread measures used for these bonds, notably the G-spread over an interpolated government yield, the I-spread over swaps and the option-adjusted spread where the issue is callable. Finally, be able to argue why the rating is a lagging summary of a credit rather than a substitute for analysis of leverage, coverage, liquidity and the issuer's willingness to defend its rating.