"Chapeau" is a French term signifying a hat or other covering for the head. In heraldry, it is used as a mark of ecclesiastical dignity, especially that of cardinals, which is called the red chapeau. It is worn over the shield by way of crest, as mitres and coronets are.
A chapeau is flat, very narrow atop, but with a broad brim, adorned with long silken strings interlaced; suspended from within with rows of tassels, called by the Italians focci, increasing in number as they come lower. The hat was given to them by Innocent IV in 1250, but was not used in arms till the year 1300. Until that time, the cardinals were represented with mitres.
The crest is born on the chapeau; and by the chapeau the crest and coat are separated; it being a rule that no crest must touch the shield immediately.
Some forms of bicorne were designed to be folded flat, so that they could be conveniently tucked under the arm when not being worn. A bicorne of this style is also known as a chapeau-bras or chapeau-de-bras.