<p>Proteins/polypeptides are a large class of organic/biochemical/biomedical related molecules most simply and most generally described by the generic structure NH₂CH(R¹)CONHCH(R²)CONHCH(R³),,, NHCH(R^{some large number})COOH, or more properly as the corresponding zwitterion. In these species, R¹, R², R^{some large number} are arbitrarily chosen from a well-defined collection of some 20 affixed groups. The archetypal example is polyglycine, the related “shorter” glycine, diglycine … hexaglycine. For these species, all of these R groups are H and much of their understanding has come from calorimetric determinations of their enthalpies of formation, and more recently high-level quantum chemical calculations. In the current study, we ask the question given as the title of this paper “If polyglycine is the polymer, then what is the monomeric repeating unit)?” Three natural choices are given, − CH₂–CO–NH − , − NH–CH₂ − CO–, or − CH₂–NH–CO − . From the analysis of the energetics of the related dimer, 2,5-diketopierazine, we demonstrate that these choices are in fact equivalent.</p>