London.crafts wiki

Mattress stitch is a method of joining two pieces of stockinette invisibly. See Lucy Neatby's mattress stitch tutorial for a photo of just how invisible this is.

Mattress stitch does rely on any increases or decreases being at least two stitches in from the sides of the pieces. It also hides one stitch at each edge, as these are drawn to the wrong side to form the seam, so you need to be aware of this if doing fancy colourwork.

This stitch is rougher than many on the yarn used to do the joining, as it's pulled through the fabric so many times, so it's best to use short lengths rather than one long one.

See Also: Knitting Tips