The St. Birgitta’s coif/cap (also, Bridget, Birgitte, Brigid), currently owned by the conservation department of Sweden’s Riksantikbarieambete (National Heritage Board) is attributed to the fourteenth-century century Swedish saint of that name. The original linen cap was at some point, most likely in the sixteenth-century, covered with silk taffeta which has since been removed by the conservation department. According to the article in Clothing and Textiles the linen has not been carbon dated, so while it most likely dates to the 14th century, the exact age is unknown.
The original linen cap is two halves joined with a complex herringbone stitch similar to a modern faggoted seam, from the front edge to about 2 inches above the bottom edge. The bottom edge is then gathered into the binding strip that runs across the front edge and then continues on as the ties that help to secure the cap on your head.
Images of similar looking caps can be found in many 13th and 14th century manuscripts. While most images of women wearing only a cap during the day appear to be servants, it is not inconceivable for many of the upper class women to be wearing one under their wimple and veil as there are also images of women sleeping in what appears to be a similarly shaped cap.
Through experimentation I, and many women that I have talked to, have discovered that fastening the wimple and veil to the St. Birgitta’s cap does help everything to stay in place far more easily and comfortably, for hours at a time.