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FRM Exam Part II · An Introduction to Securitisation

Tranching, Subordination and Credit Enhancement in Securitisation

Updated 11 October 2026 · Fact-checked

Tranching splits the cash flows and losses of a pooled asset portfolio into slices ranked by seniority. Subordination means junior tranches absorb losses first, protecting senior tranches. Credit enhancement adds further cushions: overcollateralisation, excess spread and reserve accounts. To solve questions, apply losses from the bottom of the structure upward.

Understand Tranching, Subordination and Credit Enhancement

A securitisation pools loans and sells claims on the pool's cash flows to investors. Instead of selling one identical claim, the structure issues several tranches with different priority. This is tranching.

The senior tranche is paid first and takes losses last. The mezzanine tranche sits in the middle. The equity tranche (first-loss piece) is paid last and takes losses first. This ranking is subordination. A tranche's attachment point is the pool loss at which it starts to lose. Its detachment point is the loss at which it is wiped out. Its thickness is detachment minus attachment.

Losses are allocated bottom-up (the loss waterfall). Pool losses first erode the equity tranche, then mezzanine, then senior. A senior tranche loses nothing until pool losses exceed its attachment point. Because tranches are thin or thick slices of the same pool, the equity tranche is highly risky and earns a high coupon. The senior tranche is much safer and earns a low coupon. Tranche risk depends on the loss distribution of the pool, not just on its expected loss.

Credit enhancement improves the credit quality of tranches beyond subordination. Overcollateralisation means the pool's principal exceeds the notes issued, so the excess absorbs losses first. Excess spread is the interest earned on the pool minus the coupons, fees and servicing costs paid out. It can cover current-period losses or be trapped to build reserves. A reserve account is cash set aside to cover shortfalls. Excess spread is only available if the pool actually generates it, so it weakens when defaults rise.

Default correlation matters. If pool defaults are highly correlated, the chance of very large losses rises. That hurts senior tranches most and can make equity tranches relatively less risky. Senior tranches are therefore exposed to systematic risk, which is a key lesson from the subprime crisis.

Key formulas to remember

Tranche thickness
Thickness = Detachment point − Attachment point
Expressed as a percentage of total pool notional.
Tranche loss
Tranche loss = min[ max(Pool loss − Attachment, 0), Detachment − Attachment ]
Work in currency or in percent of pool consistently. Tranche loss rate = Tranche loss ÷ Thickness.
Overcollateralisation
OC amount = Pool principal − Notes issued; OC ratio = Pool principal ÷ Notes issued
The OC amount sits below all note holders and absorbs losses first.
Excess spread
Excess spread = Pool interest income − (Tranche coupons + Fees + Servicing costs)
Often shown as an annualised percentage of pool balance. It is not guaranteed.
Subordination level
Credit enhancement for a tranche = Sum of all tranches below it ÷ Pool size
Equals the tranche's attachment point when the equity piece is included.

How to solve Tranching, Subordination and Credit Enhancement questions

Use this method for any question on tranche losses, enhancement or risk comparison.

  1. 1Write the capital structure from the bottom up: equity, mezzanine, senior. Note each size in currency or percent of the pool.
  2. 2Compute attachment and detachment points by adding tranche sizes from the bottom.
  3. 3Find total pool loss: pool balance × default rate × (1 − recovery rate), unless given directly.
  4. 4Add any enhancement that sits before note losses: excess spread used or reserve account balance, and overcollateralisation.
  5. 5Allocate the net loss bottom-up. Fill each tranche up to its thickness before touching the next.
  6. 6Compute each tranche's loss rate as loss ÷ tranche size, and what remains.
  7. 7Interpret: which tranche is impaired, how much cushion the senior tranche has left, and what changes if correlation or losses rise.

Quickest way: Waterfall subtraction

When to use it: Use when a question gives tranche sizes and a pool loss and asks which tranches lose money or how much.

  1. List attachment points: equity 0, then each next tranche starts where the one below ends.
  2. Subtract the enhancement (reserve, excess spread used) from the pool loss first.
  3. Compare the remaining loss to each attachment point. A tranche loses only the part above its attachment, capped at its thickness.
  4. Divide by thickness for loss rate. Check the answer lies between 0% and 100%.

Common mistakes in Tranching, Subordination and Credit Enhancement

  • Allocating losses pro rata across tranches.

    Students treat tranches like shares of equal standing.

    Fix: Losses go bottom-up in strict order. Senior loses only after all junior tranches are exhausted.

  • Measuring tranche loss rate against the whole pool instead of the tranche.

    The loss is quoted in pool terms, so the denominator is not switched.

    Fix: Tranche loss rate = loss in the tranche ÷ tranche size. A 2% pool loss can wipe out a 2% equity tranche entirely.

  • Treating excess spread as guaranteed protection.

    It is listed as credit enhancement, so it looks like fixed capital.

    Fix: Excess spread depends on pool interest income and falls when defaults or prepayments rise. It is a flow, not a fixed cushion.

  • Confusing overcollateralisation with a reserve account.

    Both are extra protection below the notes.

    Fix: OC is extra pool assets beyond the notes issued. A reserve account is separate cash, often funded from excess spread or upfront.

  • Assuming higher default correlation always makes every tranche riskier.

    Correlation is linked loosely with 'more risk'.

    Fix: Higher correlation fattens the tail, so senior tranches become riskier. Equity tranche risk can fall because expected loss is unchanged but losses become more clustered.

  • Thinking a senior tranche's rating removes systematic risk.

    Senior tranches seem nearly risk-free.

    Fix: A senior tranche can lose heavily in a severe systemic downturn when pool losses pass its attachment point.

Worked examples

Example 1

A pool of $500 million is tranched as: equity $25 million, mezzanine $75 million, senior $400 million. Pool defaults are 10% of the pool, with 40% recovery. Find the loss to each tranche and each tranche's loss rate. Ignore excess spread.

Show the solution
  1. Pool loss = 500 × 10% × (1 − 40%) = 500 × 0.10 × 0.60 = $30 million.
  2. Attachment points: equity 0 to 25; mezzanine 25 to 100; senior 100 to 500.
  3. Equity absorbs the first $25 million: loss = $25 million, loss rate 100%.
  4. Remaining loss = 30 − 25 = $5 million. Mezzanine absorbs $5 million out of $75 million: loss rate = 5 ÷ 75 = 6.67%.
  5. Senior loss = $0, since pool loss of $30 million is below its $100 million attachment.

Answer: Equity loses $25 million (100%), mezzanine loses $5 million (about 6.67%), senior loses nothing.

Example 2

A securitisation holds a pool of ₹1,000 crore and issues notes of ₹950 crore. Pool interest income is ₹80 crore a year. Note coupons are ₹55 crore and fees and servicing are ₹10 crore. Credit losses in the year are ₹30 crore. Assume excess spread is first used to cover the year's losses and the remainder is unabsorbed by the notes. State the OC amount, the excess spread, and the loss to the notes if OC is then used.

Show the solution
  1. OC amount = 1,000 − 950 = ₹50 crore.
  2. Excess spread = 80 − (55 + 10) = ₹15 crore.
  3. Losses after excess spread = 30 − 15 = ₹15 crore.
  4. Apply the ₹15 crore to OC: OC falls from ₹50 crore to ₹35 crore.
  5. Noteholders suffer no loss because OC is not exhausted.

Answer: OC is ₹50 crore, excess spread is ₹15 crore, and the notes take no loss. OC falls to ₹35 crore after absorbing ₹15 crore.

Exam tips

  • Draw the waterfall as a vertical stack in your rough work. Most questions become subtraction once the stack is drawn.
  • Check the denominator when asked for a loss rate: tranche size, not pool size.
  • Read whether the question treats excess spread as available. If it is stated, use it before OC and note losses.
  • For correlation questions, think about the tail: higher correlation hurts the senior tranche and changes the equity tranche less, or can help it.
  • Expect case wording on the subprime crisis: thin mezzanine tranches, rating reliance and senior tranches hit by systemic losses.

Practice questions from An Introduction to Securitisation

Tranching, Subordination and Credit Enhancement in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Tranching, Subordination and Credit Enhancement: frequently asked questions

What is the difference between tranching and subordination?

Tranching is the act of splitting the pool's claims into slices. Subordination is the ranking rule that makes junior slices take losses before senior ones. Subordination is what gives the senior tranche its protection.

What is excess spread in a securitisation?

It is pool interest income left after paying tranche coupons, fees and servicing costs. It can absorb current losses or be trapped in a reserve account. It is not fixed and shrinks when defaults rise.

How are losses allocated across tranches?

Losses are applied from the bottom up. The equity tranche absorbs them first, then mezzanine, then senior. A tranche is impaired only when pool losses exceed its attachment point.

Equity tranche vs senior tranche: which is riskier?

The equity tranche is riskier because it takes first losses and has the highest chance of impairment. It earns the highest return. The senior tranche has low default risk but is exposed to extreme systemic losses.