This technique yields an almost invisible seam. Can you find the seam in this photo?
This method is very similar to the mattress stitch, however, it has been slightly modified to fit within the context of the Tunisian simple stitch.
Note: Pull yarn taut after every stitch, so the seaming yarn is invisible from the front of the fabric and no extra stitches are created.
Starting at outer edge of piece, use tail of the last color used and thread it through your yarn needle.
Pass the needle from front to back under both strands of the first stitch of the first row on the left side and pull the yarn through. [Image 1]
Then pass needle from front to back under back loop of first stitch of other side, out of which same yarn tail is emerging. [Image 2]
Pull the tail snugly, until the “Vs” along the edges come together with the same tension as all other stitches and no additional stitch is created.
On the back of the square, make one anchoring stitch to keep edge from being pulled too tightly as you work the seam. This will prevent a pucker along the perimeter of the piece.
From the back of the fabric, insert the needle under the vertical bar of the next stitch and pull the yarn through. [Image 3]
Find the corresponding bar on the other side of the seam and move the needle from right to left under the bar, being careful not to split the yarn. [Image 4]
Going back up to the first edge, insert the needle from front to back into the same place from where it emerged. Then, move the needle horizontally, under the fabric, until it emerges just to the left of the next vertical bar. [Image 5]
Pull the needle and yarn through, being careful not to tighten too much. You want the vertical bars on both sides of the seam to match the size and appearance of all other vertical bars in the entire piece. [Image 6]
Repeat this sequence for every vertical bar along the seam. Pull the seaming yarn taut until it becomes invisible. Note that all the vertical bars are touching each other and almost identical to the surrounding bars in appearance. [Image 7]
The working yarn will now be on the back of the piece. Weave in all ends and cut off the excess yarn.
To see a video demonstration, visit my YouTube channel:
https://www.youtube.com/watch?v=djU4u17mPmM, go to 3:28.