This decorative stitch combines two parallel lines of chain stitches- the big ones on the bottom line and the tiny ones on the top. This combination results in  ‘crests’ which contributes to the name. It a great stitch to be worked on borders and combined with other stitches in different colors.

I will work this stitch between two parallel stitch lines. You need to know the Chain Stitch to understand this tutorial. 

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Fig 1: Start from the right end of the bottom stitch line. Take the needle in an out as shown and loop the thread around to make a chain stitch.  Fig 2: Next, move to the top stitch line and pluck a little fabric, as shown. Keep it towards the left of the earlier chain made. 
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Fig 3: Loop the thread around the needle, as shown.  Fig 4: Pull out the needle and you will get a formation like this.
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Fig 5: Pass the needle under thread. Be careful not to pluck the fabric under while you do it. Pull the thread all the way gently.  Fig 6: Now, get ready to make the second loop for the chain. Take the needle in through the previous chain and out, as shown. Loop the thread again.
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Fig 5: Pull out the thread and continue by making the smaller chain on the top stitch line. Keep up with this process.  Fig 6: A finished crested chain stitch would look like this. Note the ‘crests’ that got formes.