In this article we focus on a fundamental element in the conceptual development of fraction, that is, the part-whole subconstruct. This focus is grounded in a research project with elementary teachers in which a block of three sessions was spent on fractions and where issues related to the (relativity of the) whole, or what came to be called the referent, occupied a significant space in learning as it evolved through the sessions. Through outlining how the meanings given by the teachers to the whole of a fraction unfolded with/in various tasks during the sessions, we illustrate the intricacies of its development, that is, its evolution, in order to highlight the complex ramifications that underpin its learning. In addition, we describe the fundamental role of the (relativity of the) whole in learning fractions, as we outline teachers’ engagement with other subconstruct of the fraction concept.