BACKGROUND Recalcitrant seizures after callosotomy procedures are challenging to treat. One commissure, the psalterium, has received little attention. OBJECTIVE To review the literature on this structure, and with dissection, better elucidate its morphology and relationships. METHODS Twenty adult cadaver brains underwent microsurgical dissection of the psalterium. Measurements included the size and distance from the splenium of the corpus callosum and posterior border of the hippocampus tail. Observations included the relationships between the psalterium and the vein of Galen and the fiber direction within this structure. RESULTS The psalterium was identified in all specimens. It intimately contacted the undersurface of the splenium superiorly and the velum interpositum inferiorly. It was always in the midline. Just posterior to the psalterium, the internal cerebral veins were found leaving the velum interpositum to drain into the vein of Galen. In most specimens, the psalterium fibers traveled in the same plane as the commissural fibers of the splenium. Mean width of the psalterium was 11 mm; mean length was 13 mm. Average thickness in the midline was 1.4 mm, and at the lateral edges, it was generally thicker as it joined the fornix with an average of 2 mm. In 30%, efferents from the hippocampus were arranged in the form of a sheet bilaterally rather than the normal bundle-like formation forming the posterior crura. CONCLUSION Better understanding of the commissures of the human brain, including the psalterium, might help the neurosurgeon during procedures near this structure.