Cognitive flexibility and habitual stability are essential for survival, enabling adaptation to dynamic environments while ensuring efficiency in stable conditions. A key question is how the brain reconciles these seemingly conflicting properties of flexibility and stability, given that they are unlikely to be mediated by a single neuron or system. In primates, the expanded rostral-caudal axis of the brain provides distinct basal ganglia regions specialised for these functions. Specifically, the rostral and caudal regions of the caudate have been shown to process cognitive flexibility and habitual stability, respectively. This discovery revealed key insights into basal ganglia organisation and established the principle of rostral-caudal functional organisation. This principle demonstrates that basal ganglia structures are functionally organised: rostral regions mediate flexible, updatable goal-directed behaviours, while caudal regions support stable, sustained habitual actions—a framework now recognised across most basal ganglia regions. It offers a cohesive framework for understanding how the primate brain learns, retains, and executes two opposing behavioural strategies through a parallel cortico-basal ganglia system. Parallel processing along the rostral-caudal axis not only provides a unifying framework for basal ganglia function, but also elucidates the mechanisms underlying decision-making at the circuit level and the pathophysiology of basal ganglia-related disorders.