Double bouquet cells (DBCs) are neocortical gamma-aminobutyric acid (GABA)ergic interneurons characterized by the vertical bundling of its axon, which are generally termed "bundles" or "horse-tails." Using immunocytochemistry for the calcium binding protein calbindin, we have analyzed the morphology, density, and distribution of DBC horse-tails in different cortical areas of the human cortex (Brodmann's areas 10, 4, 3b, 22, 18, and 17). Although DBC horse-tails were very numerous and regularly distributed in all cortical areas, variations were observed both in terms of morphology and density. We distinguished two major classes of DBC horse-tails: the thicker complex type (type I) that had more axon collaterals; and the simple type (type II). The density of DBC horse-tails was significantly higher in areas 17, 18, 22, and 4 than in areas 3b and 10. Moreover, the proportion of type I and type II DBC horse-tails varied in the cortical areas studied. We also examined the distribution of DBC horse-tails in frontal, parietal, and occipital areas of different mammalian species. We found DBCs to be present in carnivores but not in rodents, lagomorphs, or artiodactyls. In carnivores, relatively few DBC horse-tails can be identified and they were generally found in the occipital cortex. Therefore, there is significant variability in the morphology and distribution of DBC horse-tails in different species and cortical areas. We conclude that, although these interneurons may be an important element in the organization of cortical microcolumns in primates, this is not the case in other mammalian species.