Primate enamel is subdivided into inner enamel, having Hunter–Schreger bands, and outer enamel with all rods parallel to each other. Outer inter-rod enamel may surround each rod, lie between rows of rods, or be absent, as in the ‘keyhole pattern’, which is composed entirely of rods. One theory on the formation of the ‘keyhole’ pattern overlays the hexagonal cross-sectional shape of four or more ameloblasts over the keyhole shape of the enamel rod. This ignores the likelihood that Tomes processes have a different shape from the cell body, and also ignores the observation that paths of enamel rods sometimes diverge. Scanning electron microscopy (SEM) revealed the keyhole shape of the forming face of monkey enamel. These forming rods were arranged in stepped rows with the head regions in each row separated by the tails of the preceding row. Consequently, each forming face of a rod was surrounded on three sides by previously formed enamel. The apical surface of the Tomes process was shaped exactly like the forming rod face, permitting direct apposition of one rod and one Tomes process. The conclusion was that, in the monkey, each rod of the keyhole enamel configuration is produced by one ameloblast.