On the SIM version of the same mortgage, the borrower owes .06/365 x 100,000 = $16.44 of interest daily. On the first day of the month when the first payment is due, he owes $16.44 x 30 = $493 if the prior month had 30 days, $510 if it had 31. If he pays on the first, those are the amounts he owes.

If he pays after the first, however, he owes another $16.44 for every day he is late. If he pays on the 10th, he owes $164.40 more than if he paid on the first.

Implications For Amortization: On a 30-year 6 percent loan, the total payment of principal and interest is $599.56 for both a standard mortgage and a SIM. On the standard mortgage, $500 goes to interest as calculated above, and the remaining $99.56 to principal, reducing the loan balance by that amount. The borrower will pay a late fee for a payment beyond the grace period, but the allocation of the payment between interest and principal is not affected by when during the month the payment is made.

On the SIM, in contrast, the allocation between principal and interest depends on the day the payment is credited. Any delay in the payment raises the part going to interest and reduces the part going to principal by the same amount. If the payment in the example is late by seven days or more, interest will absorb the entire payment and there will be no reduction in the balance.

SIMs Have Late Charges: Since interest on a SIM is charged daily, there is no rationale for a late charge, but lenders impose one anyway -- because they can. A late fee on a standard mortgage is completely reasonable, but late fees on SIMs are an abuse.

SIMs Encourage Inefficient Payment Processing: Borrowers are credited for payments when the payments are posted by the lender, not when they are sent by the borrower. Every day of delay generates another day of interest income, and if the lender delays posting the payment past the penalty-free period, the borrower will be billed for a late fee as well.

This means that borrowers who want to avoid the slippery slope have to adjust their payment practices to the posting procedures of the lender. This is difficult to do unless they receive monthly statements. Not all servicers provide monthly statements.

Recognizing a SIM: You might think that the mortgage note would show whether a mortgage was a SIM, but that is not the case. I looked at a simple-interest note yesterday and it didn't have a clue. I knew it was a SIM only because the borrower and his lawyer told me it was.

Evidently this is typical. One reader reported to me that she had a standard mortgage for several years, which was then converted to a SIM when it was sold to another lender. When she complained, the lender informed her that all the mortgages they purchased were converted to SIMs if the notes did not preclude it.

The possibility that a standard mortgage with an ambiguous note can be converted to simple interest is scary. Truth in Lending is no help because lenders are not required to identify loans as SIMs. I will have some more to say about this in a forthcoming article.

The writer is professor of finance emeritus at the Wharton School of the University of Pennsylvania. Comments and questions can be left at www.mtgprofessor.com.

Copyright 2006 Jack Guttentag