Interlocking ‘Y’s (pattern)
Working vertically, fill the area with columns of running stitch four fabric threads apart. Work each running stitch over two, and under two fabric threads.
Working horizontally, make pairs of diagonal stitches slanted in alternate directions to form arrowheads. Each stitch should cover two intersections of the fabric.
The point of each arrowhead should be inline with the top of each running stitch. The direction of the arrowheads should alternate, pointing either up or down. Where they point down, they should join the running stitch.
Structure of stitch
Becky Hogg, RSN Essential Stitch Guides: Blackwork (2010)
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.143