X is said to have been derived from the biretta, a similar-looking accessory worn by Roman Catholic clergy. This itself was a development of the Roman pileus quadratus, which consisted of a superimposed square with a tump (small mound). The red biretta symbolized royal power, and was generally worn in the 14th and 15th century by what was essentially the elite of the society.
What modern day accessory did the biretta lead to?