The example shown is a particular realization of the standard construction of this combination of rosettes. In essence, the standard construction is based on arrangements of regular polygons in contact at certain shared edges; peripheral stars surrounding the rosettes are centred on the vertices of the regular polygons, and are inscribed in circles of radius equal to half the polygon edge length. In the pattern shown, the 9-rosettes are arbitrarily chosen to have parallel-sided petals, and for a standard construction this choice automatically results in identical pairs of interstitial petals between the 9-rosettes, and convergent-sided petals in the 12-rosettes. Although this combination of 9- and 12-rosettes is common throughout Islam, from Morocco to India, the theoretically exact standard construction, in the present sense, is rarely seen. In western Islam - Morocco and Moorish Spain - the standard construction was traditionally known, but both kinds of rosettes are parallel-sided.
v49