The invisible horizontal seam is used in our garter stitch and stockinette stitch cowls to join the cast on edge to bind off edge.
You will follow the path your knitted stitches take to create new sewn stitches so that this seam blends into the rest of your project. This is a great way of visualizing how knitting is just making new loops through old ones!
Mind your tension - look at your work and make sure the stitches you are now sewing match the size of the ones you've knitted!
Note that you will need the same number of stitches on your cast on and bind of edge to make this line up - if you find that you have some mystery increases in your fabric, make sure to decrease by knitting 2 stitches together every few rows before you bind off.
An alternative way of seaming that also works well for our cowl projects is a simple whip stitch.