Many models of haematopoiesis dictate that haematopoietic stem cells make an early and irrevocable choice between lymphoid and myeloid cell fates. However, recent data show that intermediate progenitors have both lymphoid and myeloid potential, thereby refuting early theories and leading to a lack of consensus in the field. In this Opinion article we present a simple pairwise relationships model of cell fate determination that does not depend on the underlying developmental branch points that in other models dictate a preferred route (or routes) to a particular cell fate.