Sequential vs Pro-Rata Principal Payment

When borrowers in a securitized pool pay down principal, that cash has to go somewhere — and which tranche receives it first is a separate design choice from which tranche absorbs losses first. Two structures dominate: sequential pay, where one tranche is paid off completely before the next starts receiving principal, and pro-rata pay, where every tranche receives principal at the same time, split in fixed proportion to its size.

How each structure behaves as the pool shrinks

Under sequential pay, all scheduled and prepaid principal goes to the senior tranche until it's retired, then to mezzanine, then to equity. Because the senior tranche shrinks fastest while the loss-absorbing tranches below it shrink only through actual losses, the senior tranche's percentage of subordination protection rises steadily through the deal's life. Under pro-rata pay, every tranche shrinks at the same percentage rate, so a tranche that started with 15% subordination below it still has roughly 15% below it years later — protection doesn't build with time.

pro-rata all tranches shrink together

later: senior fully gone, subordination = 100%

Worked example

A $1,000 million pool starts with senior $850 million, mezzanine $100 million, equity $50 million (senior subordination: 15%). After a year, $200 million of principal has come in with no losses. Under sequential pay, all $200 million retires senior, leaving senior at $650 million against an unchanged $150 million of mezzanine plus equity below it — subordination has risen to about 19% ($150m / $800m). Under pro-rata pay, the $200 million is split 85/10/5, leaving senior at $680 million, mezzanine at $85 million, equity at $42.5 million — subordination is still almost exactly 15% ($127.5m / $807.5m).

Why deals use one or mix both

Sequential pay is simpler and gives senior investors growing protection for free as the deal seasons, which is why it dominates early in a deal's life or in riskier pools. Pro-rata pay lets junior tranches get their principal back faster instead of waiting years behind a large senior balance, which junior investors prefer — but it means their protection percentage never improves with age. Many deals start sequential and switch to pro-rata once the pool has seasoned and passed specific performance tests, capturing some of each structure's benefit.

What this means in practice

Two tranches with identical starting subordination can diverge sharply in actual protection after a few years, purely because one deal pays sequentially and the other pro-rata — the payment structure is as much a part of the credit analysis as the pool's default rate.