|Single Thread Couching|
You use just one strand of a six stranded embroidery thread. Bring your thread up at the beginning of the line you want to work, take it down, and then back up at the end of the section and couch back to where you started. As you can see I've used four colours, so I worked the sample in four sections.
Simple enough if the line is straight, but to work curves you have to guess how much thread to leave on the top of the fabric when you take your needle down at the end of a section. Too little and the stitch becomes tight as you run out of 'slack' before you get back along the curve to your starting point. Too much and your thread becomes too loose as you move back along it.
I learnt this stitch from James Hunting back in April and I've used it a lot since.