Interlaced cross bar
Anchor each intersection with a small diagonal stitch, ensure the stitch lies bottom left to top right.
Use a tapestry needle to whip diagonally across the shape by taking the needle under both the couching stitch and the intersection (do not pierce the fabric).
Continue to fill diagonally across the grid leaving a gap of a single intersection between each line.
Whip diagonally at 90° across the shape. This time, rather than sliding the needle under the trellis intersection, slide it under the corner of each square just after the intersection.
Structure of stitch
The thread is whipped back and forth diagonally across the intersections of a foundation grid of basic Trellis.
Royal School of Needlework, Needlework in wartime , p.8