In the context of coupled reactions, timescales are categorized as fast if they are short in comparison to the depletion timescale (t s1 ) of the primary reaction. So far, we have not discussed the transient timescale of the indicator reaction. This timescale is a "fast" timescale for the indicator reaction that it is analogous to t c1 , the transient timescale of the indicator reaction. This timescale appears when initial conditions start on the c 2 -nullcline (i.e., in QSS), then the system may be exhibit a fast transient (analogous to t c1 ) over which the phase plane trajectory swiftly travels towards the c 2 -nullcline, especially if the QSSA holds over the timescale of interest. However, the QSSA is not necessarily valid with respect to the t s1 timescale, and it is possible to derive a timescale that must be short in comparison to both t s1 and T s2 in order to apply QSSA over the time course of the non-observable reaction when the indicator reaction is substantially slower. Carefully rescaling the mass action equation for c 2 with respect to T = t/t s1 yields