Block trellis (pattern)
Working vertically, start by filling the area with an even length running stitch, each stitch over and under two threads of the fabric.
Work down each running stitch column adding two horizontal stitches to each gap. Each stitch should be over two threads and spaced by two threads of the fabric.
Work over the shape again in the opposite orientation to complete small squares.
Structure of stitch
Identifying Block trellis (pattern)
Rows of squares interconnected by single stitches.
Various Authors, The Royal School of Needlework Book of Embroidery (2018) , p.152
Becky Hogg, RSN Essential Stitch Guides: Blackwork (2010) , p.70