Coral Stitch is a great stitch for adding texture and interest to otherwise straight or curved lines - it can be a lovely (more complicated!) substitution for Back Stitch.
Coral Stitch is created by making a small knot in each stitch - I find it easiest to work from bottom to top.
1. To start, come up at the bottom of your line. Use your non-needle hand to take hold of the thread and lie it upwards along the line.
2. Bring your needle down justto the right of your line, about a centimetre up from where you started. As you pull through, use the thumb and forefinger of your non-needle hand to catch the loop of thread. The loop should have one twist near the base - this is key to creating your knot.
3. Keeping hold of your loop, come up on the left of your line - a couple of millimetres from your previous stitch. Pull your needle through - and pass it through your loop.
4. As you tighten the thread, pull it away from you - in the direction you are stitching (this keeps the stitches from bunching up).
Repeat: hold your thread along the line, and come down just to the right; keep hold of the loop, making sure it has a twist; come up on the left of the line, and pass your needle through the loop; then pull the thread away from you.
To finish a line, pass your needle to the back, just underneath your last knot.