Quadrilateral tree pixel numbering scheme. The coarsely pixelized coordinate patch on the left consists of 4 pixels. Two bits suffice to label the pixels. To increase the resolution, every pixel splits into 4 daughter pixels, shown on the right. These daughters inherit the pixel index of their parent (boxed) and acquire two new bits to form the new pixel index. Several such curvilinearly mapped coordinate patches (12 in the case of HEALPix, and 6 in the case of the COBE QuadCube) are joined at the boundaries to cover the sphere. All pixel indices carry a prefix (here omitted for clarity) that identifies which base‐resolution pixel they belong to.