This knotted line stitch is formed by working a straight stitch and then working two stitches over it to form a knot.  Working the knot in two movements produces a very raised effect. 
Structurally, the knot is worked in the same way as Sorbello stitch, although Sorbello stitch is an isolated stitch.  The double knot also has similarities with the structure of the knots in raised chain band stitch, although the direction of the first knot is different. 
Some authors work double knot stitch with the straight stitch protruding slightly which gives a barbed effect; others work the knots close together so it resembles a braid stitch.  The stitch can also be worked as a filling stitch: rows can be worked back and forth or always in the same direction; the knots can be offset so that they fit between the knots on the previous row, or worked closely together so that the line is more solid.  If worked back and forth, the knots can be worked in the reverse direction (although this will give them a slightly different appearance).  Some authors suggest that when the knot is worked as a mirror image, and also barbed, this variation is called small Palestrina knots.

Double knot stitch was evidently in use in the first half of the 18th century as it features on a man’s linen waistcoat in the Embroiderers’ Guild (UK) collection.  The whitework waistcoat features large pulled work flowers outlined in stitches including double knot stitch.

The stitch had a renaissance in the early 20th century: an embroidery school was established in Palestrina in Italy in 1907 and il punto di Palestrina was one of the core stitches for which it became known.  English authors start to feature the stitch at this time, normally under the names double knot stitch or tied coral stitch.