It doesn't always take extra swatches. When I wrote the initial instructions for "Spine Stitch," all I did was add two stitches, one for each edge, to be knit on the right side and purled on the wrong side. It turns out to be perfect for a very neat finish.
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The diagram shows how to find the horizontal running thread between the whole selvedge stitch and the half stitch next to it. Take a tapestry needle under each running thread in turn moving from side to side. (You can start on either side.)
And here is the result. Even with a contrasting sewing yarn, it is almost invisible! The seam can be touched up a bit by using a tapestry needle to pull on any receding half stitches. Then mist the seam with water and allow to dry.
Until next time!