Thorn stitch is a decorative form of couching where a long thread is held down by an off-centred cross. It is often used to depict thorny stems or ferns, hence its name. Any type of embroidery thread can be used to work this stitch: both couched and couching elements can be worked with a single thread or you could use a different type/colour of thread for the pairs of crossing stitches.
N.B. this is a different version of thorn stitch from Thorn stitch (Mountmellick).
Bring the needle up and work a long straight stitch. Then bring the needle up near the end of the first long stitch.
Work a pair of diagonal stitches so that they cross over the first long stitch.
Structure of stitch
S. F. A. Caulfield, Blanche C. Saward, The Dictionary of Needlework (1882) , p.195
Mrs Archibald Christie, Samplers and Stitches (1921) , p.19–20
Mary Thomas’s Dictionary of Embroidery Stitches (1934) , p.198
Jennifer Campbell, Ann-Marie Bakewell, Guide to Embroidery Stitches (2004) , p.108
Sarah Whittle, The Needlecraft Stitch Directory (2012) , p.107