The knot code is a close-kept secret of the Women's World. We use it to communicate, sisterkin to kin.
– Trishien, Honor's Paradox, "Chapter VI: History Lessons", I
The knot-stitch code, as the name implies, is a way of sending encoded messages in embroidery stitches on cloth. It is a third circle secret in the Women's World. While knot-stitch code is used for multiple uses, communication between sister-kin is it's primary use, hence the great secrecy surrounding it.
Jame suspects that the Kendar also use knot-stitch code, though likely a different version that the one Highborn ladies use.
↑Seeker's Mask, "Part I", II — " 'I never said a-anything about using knot stitches as a code!' she wailed, clutching her shredded sampler. 'That's a third circle secret.' "
↑Seeker's Mask, "Part I", II — "stitching a letter to a five times great granddaughter"
↑Honor's Paradox, "Chapter VI: History Lessons", I — " 'We use it to communicate, sisterkin to kin.' "
↑Honor's Paradox, "Chapter XIX: Challenge", III — "She ran her hands over the glimmering sleeves, feeling the texture of silken stitches under her fingertips. Did the Kendar also use knot codes? She felt instinctively that they did,"
↑Bound in Blood, "Chapter IV: In the Moon Garden", II — "She ran her fingertips over the raised dots."
↑Seeker's Mask, "Part I", II — "glance at Cattila's Ear, who sat in the shadows knitting their conversation into a scarf which she would subsequently dispatch to her mistress."
↑Seeker's Mask, "Part I", II — "She felt her friend's hand on her shoulder. The fingers spoke to her with quick, deft changes of pressure."
↑Seeker's Mask, "Part I", V — "A hand touched her shoulder. *Grandmother-kin,* said the fingers, with that special emphasis that indicated an embrace."