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The FitzHugh-Nagumo model represents an excitable cell by a fast activation variable and a slower recovery variable. A common nondimensional form is $\dot{V}=V-V^3/3-W+I$, $\dot{W}=\epsilon (V+a-bW)$. Its cubic activation response and recovery feedback can produce an excitable resting state or sustained oscillations through bifurcations. Particular reductions and travelling-wave equations can have additional state variables; their signs and parameter conventions must be stated separately.

A planform is the horizontal spatial structure of a pattern, often described by the Fourier modes active at a bifurcation. For example, one pair of opposite wavevectors gives parallel convection rolls, while two equal-amplitude perpendicular pairs give squares. A planform includes the relative amplitudes and phases needed to distinguish spatial symmetry types.