The beret is a traditional headwear whose structure can be described through a very simple geometry: the relationship between an outer volume and an inner opening that determines its fit on the head. From this idea a parametric construction system is developed based on head circumference, the cranial ovality index and the relationship between the top plate, the brim width and the inner opening. This system makes it possible to generate beret patterns adaptable to different sizes, cranial proportions and design variations.