We apply an Input/Output approach to the analysis of the so-called traditional standardized amortization plans (STAP). The approach is based on a clear distinction between Input/trigger and Output of a STAP. The Output is a set of four vectors obtained by applying a set of transformation matrices to an appropriate Input/trigger vector. The main result of the paper is that, by this approach, any STAP reveals a dual nature through the introduction of the twin trigger theory.
