Q1: Why deploy N wavefront sensors on a three mirror anastigmat (TMA) and not N + 1? Q2: Why measure M Zernike coefficients and not M + 1? Q3: Why control L rigid body degrees of freedom (total) on the secondary and tertiary and not L + 1? The usual answer: “We did a lot of ray tracing and N, M, and L seemed OK.” We show how straightforward results from aberration theory may be used to address these questions. We consider, in particular, the case of a three mirror anastigmat.