Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits—bauplan fidelity and elaboration of design—are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the “Hinge and Caps” model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model. J. Exp. Zool. (Mol. Dev. Evol.) 310B:315–335, 2008. © 2007 Wiley-Liss, Inc.