A subdivision of the diencephalon located ventral to the thalamus, medial to the internal capsule, and lateral and caudal to the hypothalamus. It consists of the subthalamic nucleus, sensory fiber tracts terminating in the thalamus, fibers from the cerebellum, corpus striatum, midbrain motor nuclei proceeding to the thalamus, and forward extensions of the zona incerta, red nucleus, and substantia nigra.

The subthalamus, or prethalamus (formerly described as ventral thalamus), is part of the diencephalon.