This is Swatch 4 - Single Rib (K1P1), a seaming exercise. I'm not all that pleased with it. I seamed it in the manner described in the Seaming article in "Cast On" magazine, and it is not my usual method. The article suggests using an even number of stitches and starting each piece of ribbing with a knit stitch so that you end up seaming a knit stitch and a purl stitch together (leaving the ribbing pattern intact), supposedly hiding the seam "in the ditch" of the two stitches. Because the magazine article so strongly recommended this method I went with it, but I'm having second thoughts.
I normally seam ribbing by starting with an uneven number of stitches, starting and ending both pieces with a knit stitch and seaming up the centre of these edge stitches (leaving the ribbing pattern unbroken). I find it makes a finer seam (less seam bulk, especially at cuff edges) and it still turns out neat and tidy. The "Cast On" magazine article says that the seam turns out messier, but I'm not sure that it does. I think this might be worth a bit of additional experimentation.
For readers' further consideration (are you still with me?), Maggie Righetti suggests a third method in her book "Sweater Design in Plain English". She says to add an additional knit stitch so that your single rib ends in K2 at one end. This lets you seam with a full stitch, but gives you that knit/purl "ditch" seam.