The wear of a watch cap in the Navy is describe by the United States Navy Uniform Regulations. These regulations describe the color and material the watch cap is allowed to be made of, as well as the uniforms it is allowed to be worn in. It also describes where and when the watch cap is allowed to be worn.
The watch cap is a tight-fitting knit cap. It is similar to stocking caps worn by civilians in the winter except that it does not have any ornamentation adorning the hat. The watch cap predates the Navy and has been worn by sailors for more than a hundred years. It's simplicity and flexibility make it an ideal uniform item in a maritime environment.
According to the Navy Uniform Regulations, the watch cap must be closely knitted and made out of navy blue or black wool. It must be between 11 to 12 inches long, 8 1/4 inches wide at the bottom and possess a 5/8 inch border. Unlike many uniform items, watch caps meeting these physical requirements are readily available and come in different weights for different degrees of warmth.
The Navy Uniform Regulations also describe how the watch cap is to be worn. The watch cap must be worn so there is a double fold that is approximately 2 1/2 to 3 inches wide. The regulation also mandates that no rank or insignia is to be worn on the watch cap and that it is to be worn only while outdoors.
The watch cap is the only type of head gear allowed to be worn while on an active flight line. It is also able to be worn with nearly any uniform combination. In some circumstances, such as deployment, the watch cap is allowed to be worn at only certain times of the year, typically November to March.