The term area 33o refers to the part of area 33 located between area 24ab and the corpus callosum in the anterior cingulate cortex of the human ( Vogt-2012 ) and between area 24a and corpus callosum in the rat ( Vogt-2013 ). It has no equivalent in the macaque ( Vogt-1987 ) or the mouse ( Vogt-2013 ). Many reports, e.g., Vogt-2012, refer to it as 'area 33'. In NeuroNames it is designated 33o to avoid ambiguity with the superstructure area 33, which has other parts besides 33o.