PURPOSE. To investigate the anatomic basis of the optic disc margin in the normal monkey eye by colocalizing optic disc photographs to three-dimensional (3D) histomorphometric reconstructions of the same optic nerve head. METHODS. Optic disc photographs from 28 normal monkey eyes were overlaid onto 3D central retinal vessel reconstructions generated as part of postmortem optic nerve histomorphometric reconstructions for each eye. Within each reconstruction, the Bruch's membrane opening (BMO) was delineated. Alignment was achieved by matching the clinical vessel outline to the vessel reconstruction with parallel viewing software. An experienced observer viewed stereophotographs and marked the disc margin onto clinical photographs with custom software. Alignment of the delineated disc margin to the histomorphometrically defined BMO was qualitatively assessed within each image. RESULTS. In 20 eyes, BMO aligned well to the disc margin delineation. In four eyes, alignment improved after repeated colocalization. Careful review of the histomorphometric reconstructions identified that in most cases Bruch's membrane extended beyond the termination of the border tissue of Elschnig, most substantially in the superior and nasal sectors. Misalignments could be explained by inaccurate BMO marking or where Bruch's membrane terminated externally to the inferior edge of the border tissue; this latter structure aligned to the disc margin. CONCLUSIONS. BMO was a clinically detectable entity and represented the disc margin in most eyes in this study. The 3D architecture of the border tissue combined with the presence of an overhang of Bruch's membrane makes an important contribution to disc margin anatomy. © Association for Research in Vision and Ophthalmology.