TL;DR. A credit rating is an opinion on how likely a borrower is to pay you back. Three private companies — S&P Global Ratings, Moody’s Investors Service, and Fitch Ratings — issue most of them, using letter-grade scales that split the world into two very different halves at BBB−/Baa3: investment grade above, high yield (junk) below. That single notch matters more than any other line in fixed income, because it controls who is allowed to buy the bond and what it will cost to issue.
What a credit rating actually is
A credit rating is a forward-looking opinion about the ability and willingness of an issuer to meet its financial obligations in full and on time. It is not a recommendation, not a guarantee, and not a probability. Agencies are explicit on this point — the rating is an opinion, and each notch corresponds to a broad expectation of relative default risk, not a hard number.
In the United States, the credit rating industry is regulated by the SEC through the Nationally Recognized Statistical Rating Organization (NRSRO) framework created by the Credit Rating Agency Reform Act of 2006. As of 2026, the SEC lists 11 registered NRSROs, but three of them — S&P, Moody’s, and Fitch — account for the overwhelming majority of outstanding ratings globally. That concentration is why the market usually just says “the agencies.”
Ratings apply to two related but distinct things:
- Issuer ratings assess the general creditworthiness of a company, government, or structured vehicle.
- Issue ratings assess a specific bond or loan, and can differ from the issuer rating based on seniority, collateral, and covenants. A senior secured loan from a Ba2 issuer might be rated Ba1; its subordinated bond might be Ba3.
The scales, side by side
All three of the Big Three agencies use letter-grade scales with the same structural break at the fourth rung down. S&P and Fitch use plus/minus modifiers within each letter category; Moody’s uses the numbers 1–3, with 1 being the strongest inside a bucket.
| Moody’s | S&P / Fitch | Category | Typical interpretation |
|---|---|---|---|
| Aaa | AAA | Investment grade | Prime; strongest capacity to pay |
| Aa1 / Aa2 / Aa3 | AA+ / AA / AA− | Investment grade | High grade; very strong |
| A1 / A2 / A3 | A+ / A / A− | Investment grade | Upper medium grade; strong |
| Baa1 / Baa2 / Baa3 | BBB+ / BBB / BBB− | Investment grade (lowest) | Lower medium grade; adequate |
| Ba1 / Ba2 / Ba3 | BB+ / BB / BB− | High yield / junk | Speculative; substantial risk |
| B1 / B2 / B3 | B+ / B / B− | High yield | Highly speculative |
| Caa1 / Caa2 / Caa3 | CCC+ / CCC / CCC− | Distressed | Very high credit risk |
| Ca | CC | Distressed | Currently highly vulnerable |
| C | C | Distressed | Extremely speculative; near default |
| — | D | Default | Payment default has occurred |
Why the BBB−/Baa3 line matters
The investment-grade cutoff is not just a semantic label. It is baked into hundreds of pieces of regulation, prospectus language, and internal risk mandates:
- Insurance companies hold most of their fixed-income portfolios in investment grade, because state regulators (via the NAIC) assess higher capital charges against lower-rated paper.
- Money market funds and many bond mutual funds are prospectus-limited to investment grade.
- Bank Basel III risk weights for corporate exposures step up sharply below investment grade, raising the capital cost of holding the bond.
- Index inclusion: the Bloomberg US Aggregate Bond Index is investment-grade only. A bond that gets downgraded to high yield is mechanically sold by every fund benchmarked to the Agg.
- Repo eligibility and central-bank collateral schedules lean heavily on investment-grade paper.
The mechanical selling from index-tracked money is why a fallen angel — an issuer downgraded from investment grade to high yield — typically sees a spread widening well beyond what the one-notch change in default probability would justify. Everyone who has to sell hits the bid at the same time.
What each rating implies for default risk
Ratings are ordinal (Aa is safer than A), but the agencies publish annual default studies that map each rating to a historical average cumulative default rate across horizons. The exact numbers move year to year, but the shape is remarkably stable: default risk rises slowly across investment grade, then explodes across high yield.
Two things are worth internalizing. First, the jump from Baa2 to Ba2 is not one notch of risk — it is roughly a fourfold increase in five-year default probability. Second, at the bottom of the scale, single-B credits historically default about one time in three within five years. That is not a tail event; it is the base rate.
Ratings and spreads: what the market pays
Investors are compensated for taking credit risk through the option-adjusted spread (OAS) — the extra yield a corporate bond offers over a comparable Treasury, after adjusting for embedded options. Spreads compress in benign environments and blow out in recessions and credit shocks, but the ordering by rating category is consistent.
At the September 2026 snapshot, the entire investment-grade universe was priced at just 81 basis points over Treasuries — historically tight. High yield sat at 266 bps, also tight relative to the 500–800 bp range typical of recessionary stress. Ratings do not determine spreads — the market does — but the rating is the starting anchor and the reason each cohort clears at a different yield.
How the rating process actually works
The dominant business model is issuer-pays: a company that wants to issue a bond hires one or two agencies to rate it, and pays a fee that scales with issue size (typical range 3–7 basis points of principal). The obvious conflict of interest — the client pays the grader — is the reason the 2006 Reform Act, and subsequent Dodd-Frank rules, put NRSROs under SEC supervision with mandated conflict-of-interest disclosures and separation of rating analysts from commercial teams.
Once engaged, the process typically runs:
- Analyst assignment. A lead analyst is assigned based on sector and geography.
- Information request. The agency receives audited financials, forecasts, debt schedules, covenant terms, and a management presentation.
- Analytical review. The analyst applies a published sector methodology — quantitative scorecard plus qualitative overlay for management quality, industry position, ESG factors, and event risk.
- Rating committee. A committee of senior analysts votes on the rating. Individual analysts do not assign ratings on their own; the committee is the decision-maker.
- Communication and appeal. The issuer is notified. Under standard practice they can appeal with new information before the rating is published.
- Surveillance. The rating is monitored continuously and formally reviewed at least annually. Between reviews, agencies can change the outlook (Positive, Stable, Negative), place the rating on Watch for a specific event (like a pending acquisition), or take a direct rating action.
Common mistakes people make about ratings
- Treating a rating as a probability. Aa is not “99.7% safe.” It is an opinion about relative risk, calibrated to be roughly stable across cycles but not to a specific numeric default probability.
- Assuming the Big Three always agree. Split ratings are common. A bond rated Baa3 by Moody’s and BB+ by S&P is a “crossover” credit, and different indexes handle it differently (some average, some use the middle of three).
- Confusing issuer and issue ratings. The rating you see on a bond is the issue rating, which reflects seniority and security — it can be one or two notches different from the issuer.
- Forgetting the 2008 lesson. Structured products (RMBS, CDOs) rated AAA in 2006 defaulted at rates orders of magnitude higher than corporate AAA. Ratings on complex instruments are only as good as the underlying model assumptions.
- Reading downgrades as news. By the time a downgrade lands, the market has usually moved — spreads widen first, on rumor and earnings, and the agency catches up.
Related concepts and further reading
If you found this useful, the natural next reads on ECMSource cover the mechanics that ratings feed into: investment grade vs high yield, bond pricing and duration, credit default swaps, and CLOs and structured credit.
Sources
- SEC Office of Credit Ratings — Current NRSROs list
- SEC Office of Credit Ratings — overview and annual reports
- FRED — ICE BofA US Corporate Index Option-Adjusted Spread
- FRED — ICE BofA US High Yield Index Option-Adjusted Spread
- S&P Global Ratings — rating scale definitions
- Moody’s Investors Service — rating definitions and default studies
- Fitch Ratings — rating definitions
Disclosure: This article is for informational purposes only and is not investment advice.