One of the factors used to establish the pool accrual rate for an ARM MBS pool on each interest rate change date. For stated-structure ARM MBS pools, it is the difference between the lowest mortgage margin in the pool and the sum of the guaranty fee and the minimum servicing fee. For weighted-average ARM MBS pools, the MBS margin may be a fixed margin that the lender specifies or a weighted-average margin. A “fixed” MBS margin is attained by varying the servicing fee for individual mortgages to equalize the differences in their mortgage margins. A “weighted-average” MBS margin is attained by reducing the various mortgage margins by the applicable guaranty fee and a fixed servicing fee that the lender specifies, thus developing a different MBS margin for each mortgage.