Guide

Credit rating agencies explained

Harbor Capital's investment-grade corporate sleeve screened holdings by a simple rule: keep everything rated BBB- or higher at any of the Big Three agencies. In April, a regional utility issuer carried BBB from S&P and Baa2 from Moody's with a negative outlook on both. The portfolio committee treated it as core IG paper. Three weeks later, S&P cut the issuer to BB+, a fallen angel crossing the IG/HY cliff. The bond price dropped 11% in two sessions as index funds forced to sell met a thin high-yield bid. The sleeve's “IG-only” label had been a snapshot, not a risk forecast.

Credit rating agencies — principally Moody's Investors Service, S&P Global Ratings, and Fitch Ratings — publish forward-looking opinions on the ability and willingness of issuers to meet debt obligations. Their letter scales compress complex balance-sheet analysis into categories that pension funds, ETFs, and bank regulations use as hard gates. Ratings are not prices; they are coarse, lagging signals that interact sharply with credit spreads and market liquidity. This guide covers the agency landscape, scale translation, issuer vs issue ratings, outlook and watch language, the Harbor Capital policy refactor, a technique decision table vs CDS and spread models, pitfalls, and an investor checklist.

What credit rating agencies do

A credit rating is an agency's opinion on default risk over a stated horizon — typically focused on the next several years for corporate issuers. Agencies earn fees from issuers who request ratings (the “issuer-pays” model dominant since the 1970s), from investors who subscribe to research, and from surveillance on outstanding debt.

Ratings matter because they are embedded in rules, not just opinions:

Regulatory capital — bank and insurance capital charges often step at IG/HY boundaries.
Index membership — the Bloomberg U.S. Aggregate and many IG ETFs exclude sub-investment-grade bonds.
Mandate filters — pension and mutual fund charters frequently cap HY exposure or ban CCC names outright.
Collateral eligibility — repo desks and central banks accept only certain rating tiers as pledgeable collateral.

When a rating changes, mechanical buyers and sellers can move faster than fundamental news — especially at the BBB-/BB+ cliff where investment-grade status is lost.

The Big Three scales and how they map

Moody's uses an alphanumeric scale (Aaa, Aa1, … C). S&P and Fitch use capital letters with +/- modifiers (AAA, AA+, … D). The investment-grade cutoff is Baa3 (Moody's) and BBB- (S&P/Fitch). Everything below is speculative grade, commonly called high yield or junk.

Category	Moody's	S&P / Fitch	Typical interpretation
Prime / highest quality	Aaa	AAA	Minimal credit risk; often sovereigns and top corporates
High grade	Aa1 – Aa3	AA+ – AA-	Very strong capacity; small downgrade buffer
Upper medium	A1 – A3	A+ – A-	Strong but more cyclical than AA names
Lower medium	Baa1 – Baa3	BBB+ – BBB-	IG floor; BBB- is “fallen angel” watch zone
Non-investment grade	Ba1 – Ba3	BB+ – BB-	Speculative; higher spreads, more default cyclicality
Highly speculative	B1 – B3	B+ – B-	Material default risk; covenant and liquidity focus
Distressed	Caa – C	CCC – D	Very high or realized default; recovery trades dominate

A split rating — say Baa3 from Moody's and BBB from S&P — is common near category boundaries. Conservative mandates often use the lowest agency rating (weakest-link rule) or require two IG opinions.

Issuer ratings vs issue ratings

Agencies distinguish between the creditworthiness of the company and the risk of a specific bond:

Issuer / corporate family rating — default risk of the obligor at the holding-company or operating-company level, ignoring structural subordination.
Issue / instrument rating — risk of a particular bond or loan, adjusted for seniority, security, and guarantees.

Notching adjusts issue ratings up or down from the issuer anchor based on priority in bankruptcy. Senior secured debt may notch up; subordinated debentures notch down. A BBB issuer can issue BB-rated subordinated notes while senior bonds stay BBB. Portfolio screens that check only issuer ratings miss subordinated paper carrying extra risk and spread without a headline downgrade of the company.

Sovereign ceilings can cap corporate ratings in emerging markets: a strong local company may not be rated above the government's foreign-currency rating regardless of its balance sheet.

Outlook, watch, and surveillance language

The letter rating is a level; outlook and watch modifiers signal direction over the next 6–24 months (outlook) or the immediate 90-day review window (watch). Treating BBB- stable and BBB- negative outlook as identical is a common mistake.

Modifier	Meaning	Investor use
Stable outlook	Rating likely unchanged near term	Baseline; still monitor spreads for early warning
Positive / negative outlook	Upgrade or downgrade more likely than stable	Reduce size near IG cliff; widen spread hurdle for new buys
Rating watch (positive/negative/developing)	Active review; action probable within ~90 days	High urgency; index and mandate risk imminent
Withdrawn / NR	Agency no longer maintains coverage	Some mandates treat as ineligible; liquidity may thin

Agencies reaffirm most ratings without action; the rare downgrade from BBB- to BB+ moves markets because of forced selling, not because default happened overnight. Negative outlook on BBB- is often a stronger trading signal than the letter BBB alone.

How agencies form opinions (simplified)

Methodologies differ by sector, but corporate ratings generally weigh:

Business risk — industry cyclicality, competitive position, diversification, regulatory exposure.
Financial risk — leverage (debt/EBITDA), interest coverage, free cash flow, liquidity runway, off-balance-sheet obligations.
Recovery expectation — for issue ratings, asset coverage and subordination in default.
Management and governance — financial policy (shareholder returns vs debt paydown), acquisition appetite, transparency.

Ratings are through-the-cycle opinions: agencies try to rate survivability across a business cycle, not just the current quarter. That makes them slower than market spreads, which price near-term fear immediately. During stress, bonds often trade like HY before the downgrade letter arrives — the spread moves first; the rating follows.

Harbor Capital IG policy refactor

Harbor Capital's pre-refactor rule: hold bonds if any Big Three rating was BBB-/Baa3 or higher at purchase. No outlook filter, no weakest-link rule, no issue-level check on subordinated tranches. The fallen angel loss forced a rewrite.

The refactor:

Weakest-link IG test — both Moody's and S&P (Fitch when available) must be IG at purchase; if split, use the lower rating.
Outlook-aware buckets — BBB- stable capped at 3% of sleeve; BBB- negative outlook or on watch excluded from new buys and flagged for exit within 30 days.
Issue-level verification — subordinated and hybrid capital must be rated IG at the issue level, not just the issuer.
Spread override — if OAS widens more than 150 bps above the one-year sector median while still IG-rated, trigger analyst review regardless of outlook.
Fallen-angel protocol — pre-identified exit lines and block sizes before downgrade day to reduce fire-sale slippage.

Over the next two quarters, the sleeve avoided two additional BBB- names that migrated to BB+ before the committee would have acted under the old rule. Tracking error vs the IG index rose slightly from tighter BBB- limits, but peak-to-trough drawdown during a regional-bank credit scare improved by 80 basis points.

Technique decision table

Signal source	Best when	Limitation vs agency ratings
Agency letter + outlook	Mandate compliance, index eligibility, coarse bucketing	Lags markets; cliff effects at BBB-/BB+; issuer-pays conflicts debated
Credit spreads / OAS	Daily risk pricing, relative value within a sector	No legal mandate hook; noisy on illiquid names
CDS spreads	Default-probability view, hedging, lead indicator	Basis risk vs cash bonds; contract specificity
Internal scoring models	Proprietary portfolios, early downgrade warning	Not accepted for regulatory ratios; model risk
Covenant / financial covenants	Loan and HY structures with maintenance tests	Bond indentures often incurrence-only; less real-time

Production practice combines ratings for eligibility with spreads and CDS for timing. Neither alone is sufficient near the IG cliff.

Common pitfalls

Single-agency cherry-picking. Owning BBB from one agency while another rates BB+ violates the spirit of IG mandates.
Ignoring outlook. BBB- negative is not the same risk as A- stable even though both are “investment grade.”
Issuer rating on subordinated debt. Notching can put junior paper in HY while the company headline stays IG.
Assuming ratings predict short-term prices. Upgrades can lag spread tightening; downgrades can accelerate after spreads blow out.
Uniform treatment of BBB+ and BBB-. The bottom IG notch carries disproportionate downgrade and liquidity risk.
Neglecting surveillance after purchase. Ratings change; ETF inclusion and fund charters react mechanically.
Confusing sovereign and corporate ratings in EM where ceilings bind.

Investor checklist

Weakest-link rule across Moody's, S&P, and Fitch documented in policy.
Outlook and watch status screened at purchase and monthly surveillance.
Issue-level rating verified for subordinated, hybrid, and secured structures.
BBB- bucket size capped; negative outlook names on exit watchlist.
Fallen-angel playbook: liquidity providers, max sell size, spread trigger.
Spread or CDS override when market prices more risk than the letter rating.
Split-rating protocol defined before the trade, not after a downgrade.
Index and mandate impact modeled for one-notch moves at category boundaries.
Recovery assumptions updated when issue notching changes.
Regulatory and accounting treatment of NR/withdrawn ratings understood.
New-issue pipeline checked for rating agency momentum (watch placements).
Post-mortem on any fallen angel: outlook ignored vs surprise event.

Key takeaways

Ratings are coarse, rules-driven opinions — not market prices.
The BBB-/BB+ boundary creates cliff risk from forced index flows.
Outlook and watch modifiers often matter more than the letter at the margin.
Issue ratings and notching can diverge sharply from issuer headlines.
Pair agency scales with spread/CDS monitors for timing and sizing.