Ongezellig

translated version of the wikipedia page because it's only in dutch

A hul is a type of cap that is commonly worn in traditional Dutch clothing. Not only in Volendam and West Friesland, but also, for example, in Katwijk, on Urk, and in Spakenburg.

The hul is a small, simple cap that is worn tightly around the head. At the forehead, the hul has a few pleats. Sometimes the hul extends sideways into points. Above the hul, a hullenhoed (a type of hat) was sometimes worn, and underneath sometimes an under-cap. The hul was part of everyday clothing; on Sundays, a kap (a type of bonnet) was worn.

History
Fishermen’s wives along the North Sea coast wore simple hullen in the nineteenth century. From the end of that century onward, they became increasingly more elaborate, with embroidery on the crown and strips of bobbin lace from places such as Lille (Rijssel). The strips of lace were sometimes very wide, and in such cases, the hul was called a "boomhul."

Regional Costume

Woman from West Friesland wearing a hul with a hat on top, painting by August Allebé
The hul is, for example, worn in the traditional costume of Volendam. In that form, the points have become quite large. In some variants, the points are also bent upwards. The material, craftsmanship, and the way the hul is worn indicate the region it comes from.

Beach Hul
Beach hullen were intended to be worn on the beach to prevent the wearer’s hair from getting tangled. They were sold to tourists, including in Noordwijk. Beach hullen are generally more angular and less neatly made than the often costly hullen used in regional costumes. The crown of the beach hul consists of two halves with a seam in the middle. Straps at the bottom secure the hul to fit. The crown is made of machine-made lace.

Other Caps
Other caps worn in regional costumes include the luifelmuts, the kornet, the sluiermuts, and the mopmuts.