Ermine stitch consists of a vertical stitch crossed towards the bottom with two diagonal stitches.
The shape formed is similar to the heraldic pattern called ermine, itself based on the winter coat of the stoat.
Begin by making a vertical, straight stitch.
Bring the needle out to the left, slightly lower down from the top of the first stitch.
Take the needle down diagonally to the right, ensuring it finishes slightly above the bottom of the first stitch.
Bring the needle up on the left hand side, parallel with where the needle was just taken down and close to the very first stitch.
Take the needle down diagonally to the right hand side of the first stitch.
Repeat the stitch, filling the space as desired.
Structure of stitch
Mary Thomas, Mary Thomas’s Dictionary of Embroidery Stitches (1934) , p.84–5
Jennifer Campbell, Ann-Marie Bakewell, Guide to Embroidery Stitches (2004)