our discussion of CMOs issued by the agencies in the previous chap- ter we explained how a planned amortization class tranche can be cre- ated. These tranches are also created in HEL structures. A NAS tranche receives principal payments according to a schedule. The schedule is not a dollar amount. Rather, it is a principal schedule that shows for a given month the share of pro rata principal that must be distributed to the NAS tranche. A typical principal schedule for a NAS tranche is as follows:     Months Share of pro rata principal   1 through 36 0% 37 through 60 45% 61 through 72 80% 73 through 84 100% After month 84 300%       The average life for the NAS tranche is stable for a large range of pre- payments because for the first three years all prepayments are made to the other senior tranches. This reduces the risk of the NAS tranche contracting (i.e., shortening) due to fast prepayments. After month 84, 300% of its pro rata share is paid to the NAS tranche thereby reducing its extension risk. As an illustration, Exhibit 10.8 presents a Bloomberg screen that presents a HEL-backed deal issued by the Advanta Mortgage Loan Trust, Series 2000-2. Note that tranche A6 is the NAS tranche. More- over, tranches A2 through A5 are accelerated securities (AS) which means simply these tranches receive principal payments faster than the underlying collateral.   Open-End Home Equity Loan-Backed Securities With an open-end home equity loan (HELOC) the homeowner is given a credit line and can write checks or use a credit card for up to the amount of the credit line. The amount of the credit line depends on the amount of the equity the borrower has in the property. The revolving period for a HELOC is the period during which the bor- rower can take down all or part of the line of credit. The revolving period can run from 10 to 15 years. At the end of the revolving period, the HELOC can specify either a balloon payment or an amortization schedule (of up to 10 years). Almost all HELOCs are floating-rate loans. The interest rate paid by HELOC borrowers is typically reset monthly to the prime rate as reported in The Wall Street Journal plus a spread.