A mezzanine tranche absorbs pool losses between junior and senior layers. See how attachment, detachment and the choice of denominator turn a 15% pool loss into a 25% tranche loss.
Mezzanine tranche
A mezzanine tranche in structured credit is an intermediate layer of claims on a pool: it bears credit losses after more junior claims have absorbed their allotted losses, but before more senior claims do. This position gives a funded investor a cushion against initial pool losses while concentrating subsequent losses into a smaller amount of invested principal. “Mezzanine” describes the layer's relative priority; it does not specify a universal credit rating, coupon or guaranteed return.
In a securitisation, there may be several mezzanine classes with different priorities. The term here concerns pool-based tranches, rather than corporate mezzanine financing, which concerns a firm's financing structure.
Find the layer before measuring the loss
Consider a fictional pool with USD 100 million principal. Its claims, ordered from first to last in absorbing principal credit losses, are:
Stipulate one final unrecovered principal loss, allocated strictly in this order. All pool principal is represented by the three claims. There are no prior repayments, reserves or additional credit support, and interest, fees, timing and discounting are excluded.
The mezzanine attachment amount is USD 10 million: losses must exceed this amount before mezzanine principal starts to fall. Its detachment amount is USD 30 million: when cumulative pool loss reaches this amount, mezzanine principal is exhausted. Its thickness is the difference, USD 20 million. Expressed against the original pool principal, these boundaries are 10% and 30%; they are loss thresholds, not default probabilities.
This is subordination. Junior capital protects mezzanine principal, and mezzanine capital in turn protects senior principal. The arrangement redistributes the pool's loss among claims.
A 15% pool loss produces a 25% mezzanine loss
Let L be unrecovered pool principal loss in USD millions. In this stipulated static structure, mezzanine principal loss is
ℓM(L)=min(max(L−10,0),20).
For L=15, the first-loss claim absorbs USD 10 million and is exhausted. Mezzanine absorbs the remaining USD 5 million; senior absorbs zero. Mezzanine has USD 15 million principal remaining.
There are three different ratios:
Pool loss:15/100=15% of the original pool principal.
Mezzanine loss rate:5/20=25% of the original mezzanine principal.
Mezzanine loss as a share of pool principal:5/100=5%.
Calling mezzanine's loss “5%” without specifying the denominator would understate its own principal impairment. The allocated loss is USD 5 million; its percentage depends on which principal balance you divide by.
Mezzanine principal loss is zero through USD 10 million pool loss, rises to USD 20 million at pool loss 30, and remains 20 thereafter. Pool loss 15 gives mezzanine loss 5, equal to 25% of its 20 million principal.
Fictional static pool, all amounts in USD millions. The mezzanine layer bears losses above 10 through 30; a 15 million pool loss reduces its 20 million principal by 5 million.
The boundaries are inclusive in different ways. At exactly USD 10 million pool loss, mezzanine loss is zero. It becomes positive only above attachment. At exactly USD 30 million pool loss, mezzanine loss is USD 20 million, its entire principal; senior loss is still zero at that boundary. Further pool losses fall on senior under the stipulated order.
Our CDO tranche-loss lesson expresses the same bounded loss layer as the difference of two positive-part terms. The full lesson requires membership; a public preview introduces its scope.
A cushion is only part of the risk assessment
The layer tells you what happens given a pool loss. It does not tell you how likely that loss is, what price to pay, or what return the investment will earn. Those questions need the pool-loss distribution, recoveries, dependence between borrowers, payment timing and the contractual terms.
A modest change in pool loss can substantially change the impairment of a relatively thin tranche once its cushion is used up. The BIS analysis of uncertainty in securitisation explains why risk-estimation uncertainty can concentrate in intermediate layers. Being in the middle does not make risk “average.”
A complete deal also has rules for allocating collections, paying coupons, testing coverage and changing payment priorities. This principal-loss example supplies none of those cashflows. A quoted coupon is a contractual payment term, while realised return also depends on what is paid, when it is paid and the investment's purchase price. Read the actual priorities and credit support before treating the word “mezzanine” as a risk description.
Sources
John C. Hull, Options, Futures, and Other Derivatives, ninth Global Edition (Pearson, 2018), §8.1, pp.207–210 — simplified tranche hierarchy, principal losses and separate payment waterfalls. The pool above uses original fictional sizes and assigns no ratings.
