In order to develop experimental protocols that yield an accurate quantification of enzyme activity in a non-observable reaction, a mathematical model must be developed, conditions for its validity must be established, and timescales that characterize the lengths of transient and steady-state regimes must be approximated. Qualitatively, the most useful tool that can be employed to study the mathematical structure of a chemical reaction is phase-plane analysis; in contrast, the most influential quantitative tool is the combination of scaling and asymptotic analysis . The single-enzyme, single-substrate MM reaction is well understood in terms of scaling and asymptotic analyses , and the geometric picture of the phaseplane dynamics is also well defined . While scaling/asymptotic analysis has recently been applied to coupled enzyme reactions , a clear picture of the phase-plane geometry is lacking. The phase-plane description of a reaction is useful in that it illustrates the reaction visually as a two-dimensional play whose characters are trajectories, attracting manifolds, and fixed points. described by an asymptotically autonomous vector field . Consequently, while the phase-plane analysis of the single-enzyme, single-substrate MM reaction presents a phase-plane with fixed points and manifolds that are stationary, the phase-planes of the indicator reactions studied in this paper are best analyzed by moving nullcline analysis. In the sections that follow, we illustrate the geometric interpretation of the quasi-steady-state assumption (QSSA), and the reactant-stationary assumption (RSA) of the indicator reactions utilized in the auxiliary enzyme assay and zymogen activation assay.