Periodic patterning is a fundamental organizing process in biology. Using a feather reconstitution assay, we traced back to the initial stage of the patterning process. Cells started from an equivalent state and self-organized into a periodic pattern without previous cues or sequential propagation. When different numbers of dissociated mesenchymal cells were confronted with a piece of same-sized epithelium, the size of feather primordia remained constant, not the number or interbud spacing, suggesting size determination is intrinsic to dissociated cells. Increasing bone morphogenetic protein (BMP) receptor expression in mesenchymal cells decreased the size of primordia while antagonizing the BMP pathway with Noggin increased the size of primordia. A threshold number of mesenchymal cells with a basal level of adhesion molecules such as NCAM were sufficient to trigger the patterning process. The process is best visualized by the progressive restriction of beta-catenin transcripts in the epidermis. Therefore, feather size, number and spacing are modulated through the available morphogen ligands and receptors in the system.