This hat is a simple application of decorative increasing and decreasing in double-knitting. Six banners or flags in 3 different Celtic knot motifs run diagonally around this hat and up to a spiral at the crown.
Sizing this hat is very easy — the banners stay the same, but you add more pairs between them if you need a larger size. The larger the hat, the more dramatic the crown spiral becomes! The crenellated brim is a function of the increases and decreases, and also becomes more dramatic the more pairs are inserted between the banners.
This pattern is currently available only in the 2018 revision of Extreme Double-knitting.