FRM Exam Part II · Structured Credit Risk

CDOs, CLOs and Synthetic CDOs Explained

Updated 11 October 2026 · Fact-checked

A CDO pools debt assets and issues tranches that absorb losses in order: equity first, then mezzanine, then senior. A CLO is a CDO backed by leveraged loans. A synthetic CDO gets its credit exposure through credit default swaps, not by owning the assets. You solve questions by tracing losses through attachment and detachment points.

Understand CDOs, CLOs and Synthetic CDOs

A collateralized debt obligation (CDO) is a special purpose vehicle that buys a pool of debt assets and funds itself by selling claims on that pool. The claims are tranches. Each tranche has a different priority on cash flows and losses.

Losses hit the lowest tranche first. The equity tranche absorbs the first losses and pays the highest return. The mezzanine tranches sit in the middle. The senior and super senior tranches are hit last and pay the lowest spread. Cash flows (interest and principal) go the other way: senior first, equity last. This is the waterfall. Each tranche has an attachment point (where its losses start) and a detachment point (where it is wiped out).

A CLO is a CDO whose collateral is mainly leveraged (sub-investment-grade) corporate loans. A CDO is the broader label. Its collateral can be bonds, loans, ABS tranches or other CDO tranches. A cash CDO actually owns the assets. A synthetic CDO does not. The vehicle sells credit protection through CDS on a reference portfolio and earns the premiums. It funds only a small amount of collateral. Protection payments go to the protection buyer and reduce tranche principal as reference entities default.

A CDO squared (CDO²) is a CDO whose collateral is tranches of other CDOs, usually mezzanine tranches. Its underlying pools overlap, so it is highly sensitive to default correlation.

Compared with basic ABS, a CDO is often a re-securitisation. The collateral is itself debt, sometimes already tranched, and the structure is managed or actively designed. Tranche risk depends heavily on default correlation. Higher correlation raises the risk of the senior tranche and lowers the risk of the equity tranche. Mezzanine tranches are the most complex, and their value can move either way.

Key formulas to remember

Tranche loss: Tranche loss = min(max(Portfolio loss − A, 0), D − A); A is the attachment point and D the detachment point, both in currency units or as % of the pool. The tranche is wiped out when portfolio loss reaches D.
Tranche loss as % of tranche: Tranche loss % = tranche loss ÷ (D − A); Thin tranches show large percentage losses for small pool losses. This is leverage.
Tranche thickness: Thickness = D − A; Equity tranche has A = 0. Thickness sets how much pool loss it can absorb.
Portfolio loss: Portfolio loss = Σ (exposure × LGD) over defaulted names; LGD = 1 − recovery rate. Use it to get the pool loss before tranching.
Correlation effect: Higher default correlation → equity tranche value up; senior tranche value down; Mezzanine effect is ambiguous and depends on its location in the structure.
Synthetic CDO tranche cash flows: Premium on outstanding tranche notional; protection payment = tranche loss; Tranche notional falls as losses are written down.

How to solve CDOs, CLOs and Synthetic CDOs questions

Use this method for any CDO, CLO or synthetic CDO question, numerical or conceptual.

1Identify the structure: cash or synthetic, CDO, CLO or CDO squared. Note the collateral type.
2Write down the tranche boundaries: attachment and detachment points, and the pool size.
3Compute pool loss: defaults × exposure × (1 − recovery).
4Allocate the loss through the waterfall from equity upward using min(max(loss − A, 0), D − A).
5Convert to a percentage of tranche size if asked, and check the tranche is not above 100% loss.
6For correlation questions, decide whether the tranche benefits from clustered defaults (equity) or is hurt by them (senior).
7For synthetic structures, note that protection payments, not asset sales, cover the losses, and that counterparty risk of the protection side matters.
8State the interpretation in one line: who bears the loss and why.

Quickest way: Loss-layer shortcut

When to use it: Use for tranche loss calculations and correlation direction questions when time is short.

Draw the pool as a bar from 0 to 100%. Mark each tranche as a layer.
Fill the bar up to the pool loss. Anything above A is the tranche's loss, capped at D.
Divide by the thickness D − A for the tranche loss %.
For correlation: equity likes high correlation, senior fears it. Eliminate options that reverse this.
For cash versus synthetic: if the answer mentions CDS, premiums or protection, it is synthetic.

Common mistakes in CDOs, CLOs and Synthetic CDOs

Ignoring recovery and using exposure lost as the pool loss.
The question gives defaults and students count full notional.
Fix: Multiply each default by LGD = 1 − recovery before allocating to tranches.
Dividing tranche loss by pool size instead of tranche size.
Mixing pool-level and tranche-level percentages.
Fix: Tranche loss % = tranche loss ÷ (D − A). State the denominator before computing.
Saying higher correlation hurts all tranches.
Correlation is linked with 'more risk' in general.
Fix: Higher correlation raises the chance of very large and very small pool losses. It helps equity and hurts senior. Mezzanine depends on position.
Treating a CLO and a CDO as unrelated products.
The names sound like separate products.
Fix: A CLO is a CDO backed by leveraged loans. CDO is the wider label.
Thinking a synthetic CDO owns the reference assets.
The tranche structure looks the same as a cash CDO.
Fix: A synthetic CDO gets exposure through CDS. It has no asset purchase and carries protection-seller and counterparty features.
Assuming a senior rating means no risk in a CDO squared.
Ratings on ABS often track low default risk.
Fix: Re-securitised mezzanine collateral overlaps and is highly correlation-sensitive. Senior tranches can lose heavily in a systemic shock.

Worked examples

Example 1

A ₹/USD-neutral CDO has a USD 500 million pool. Tranches: equity 0–5%, mezzanine 5–15%, senior 15–100%. Eight bonds of USD 10 million each default with 40% recovery. What is the loss on the mezzanine tranche as a percentage of its size?

Show the solution

Defaulted exposure = 8 × 10 = USD 80 million.
LGD = 1 − 0.40 = 0.60, so pool loss = 80 × 0.60 = USD 48 million.
Pool loss as % of pool = 48 ÷ 500 = 9.6%.
Equity absorbs the first 5%, which is USD 25 million, so it is wiped out.
Mezzanine attaches at 5% and detaches at 15%. Loss in the layer = 9.6% − 5% = 4.6%, or USD 23 million.
Mezzanine size = 10% × 500 = USD 50 million.
Mezzanine loss % = 23 ÷ 50 = 46%.

Answer: The mezzanine tranche loses 46% of its principal. The equity tranche is fully lost and the senior tranche is untouched.

Example 2

A risk manager says: 'If default correlation in the underlying pool rises sharply, the equity tranche of a CDO becomes riskier and the senior tranche safer.' Is this correct? Explain.

Show the solution

Higher correlation makes defaults cluster. The pool is more likely to have either very few defaults or very many.
Very few defaults are more likely, so the chance that the equity tranche survives rises. Its expected loss falls and its value rises.
Very many defaults are also more likely, so losses are more likely to reach the senior tranche. Its expected loss rises and its value falls.
So the statement has both effects reversed.

Answer: Incorrect. Higher correlation generally makes the equity tranche less risky and the senior tranche riskier. The effect on mezzanine tranches depends on their position in the structure.

Exam tips

Always write A and D first. Most tranche loss errors come from wrong boundaries.
Check whether the question gives a recovery rate. If it does, apply it before allocating losses.
For correlation direction, memorise: equity long correlation, senior short correlation.
Spot keywords: 'CDS', 'reference portfolio' and 'premium' mean synthetic. 'Leveraged loans' means CLO. 'Tranches of CDOs' means CDO squared.
Under time pressure, eliminate options with tranche losses above 100% or below zero.

Practice questions from Structured Credit Risk

CDOs, CLOs and Synthetic CDOs: frequently asked questions

What is the difference between a CDO and a CLO?

A CDO is the general structure that pools debt and issues tranches. A CLO is a CDO whose collateral is mainly leveraged corporate loans. Other CDOs may hold bonds, ABS or other tranches.

How does a synthetic CDO work?

The vehicle sells credit protection on a reference portfolio through CDS and collects the premiums. Tranche investors absorb the losses on credit events in order of seniority. No cash assets are bought.

What is a CDO squared?

It is a CDO whose collateral consists of tranches of other CDOs, often mezzanine. Overlapping underlying pools make it very sensitive to default correlation and hard to value.

How do equity, mezzanine and senior tranches differ?

Equity takes the first losses and pays the highest return. Mezzanine takes losses after equity is gone. Senior takes losses last and pays the lowest spread.