Closed buttonhole stitch
Closed buttonhole stitch is widely-spaced buttonhole (also known as blanket stitch) alternate ‘legs’ of the stitch are angled forward or backward so that they meet and form a triangle.
For more background see the entries for Buttonhole stitch and Blanket stitch.
Closed buttonhole stitch was certainly in use in the 17th century as it features as an edging stitch on two English coifs, currently held by the Victoria and Albert Museum, London.
Working left to right. Bring the needle to the surface at the base of the stitch as if to create a diagonal stitch. Keep the thread slack.
Bring the needle up on the base line again half a stitch length behind the end of the last stitch, ensuring the needle comes up behind the last slack stitch.
Tighten the first stitch against the needle, creating a kink in the diagonal stitch. Pull the working thread to the surface. Take the needle down at the end of the first stitch, leaving the thread slack on the surface.
Bring the needle up on the base line again, half a stitch length ahead ensuring the needle comes up behind the last slack stitch and tighten against the needle.
Pull the thread through to the surface, creating a triangular shape.
Continue the sequence working left to right to complete an attractive border.
Structure of stitch
Mary Thomas’s Dictionary of Embroidery Stitches (1934) , p.21–2
Anchor Embroidery, 100 Embroidery Stitches
'17th century women’s coif ', Victoria and Albert Museum (2021). Available at: https://collections.vam.ac.uk/item/O78811/coif-unknown/ (Accessed: 19 October 2021)
'Elizabethan coif', Victoria and Albert Museum (2021). Available at: http://collections.vam.ac.uk/item/O251169/coif-unknown/ (Accessed: 24 November 2021)