Diamond trellis (pattern)
Work a line of horizontal back stitches across the design area: each stitch should be over four threads.
Fill the design area with vertical back stitches over four threads. Each stitch should sit over the centre of the initial horizontal thread to form a cross.
Alternate crosses should be surrounded by a diamond: work alternate straight and back stitches over two intersections.
Structure of stitch
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.154
Becky Hogg, RSN Essential Stitch Guides: Blackwork (2010) , p.75