Like most knit or crochet handiwork, adding fringe to a handmade scarf is more of an art than a science. There's no solid rule as to how many strands of yarn each unit of fringe should have, or how many units you should put on each end of the scarf. It's up to you to experiment until you find a combination that creates the look and feel you're looking for.
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Designing Your Fringe
No hard or fast rules exist as to how long your fringe should be or how many strands it should have. If you're not sure where to start, try making a few sets of 5-inch-long fringe, with six strands in each unit of fringe. If you're working with thinner yarn, you may need more strands to get a full look, while finished fringe made of thick yarn may use as few as two strands.
Cutting the Yarn
Cut half as many yarn pieces as the number of strands you want in the finished fringe; each piece of yarn should be twice as long as the finished fringe. So, if you want the finished fringe to be six strands of 5-inch-long yarn, you should cut three strands of yarn that are each 10 inches long.
The easiest way to cut consistent yarn pieces is by wrapping the yarn around a piece of cardboard that is as wide as you want the finished fringe to be long; so, for 5-inch fringe, you'd wrap the yarn around a 5-inch piece of cardboard. Wrapping the yarn three times, then snipping through the wrapped loops at one end, will create three lengths of 10-inch yarn.
Attaching the Fringe
To create six-strand fringe, hold your three strands of double-length yarn together and double them over, creating a loop on one end. Poke that loop through the stitch where you want to attach the fringe; then pull the ends of the gathered strands through the loop. For a scarf that's densely knit or crocheted, use a crochet hook or yarn needle to help tease the loop between the stitches. Tug on the yarn ends to snug the loop up against the scarf end, and repeat as desired.
There's no hard and fast rule about how widely you should space your fringe, but, as a general rule, leave a little space between the strands so the fringe can flow; you can always add more later.
If you decide you don't like the look of your fringe, just use a knitting needle, crochet hook or yarn needle to tease the knot open and remove the strands. If you're not concerned about saving the yarn to reuse, slide one scissors blade under the loop of the fringe knot. Snip through the loop; then pull the fringe strands out of the scarf.
Once you've removed the fringe, you may see some distortion or gapping between stitches. Except in the most extreme cases, that's easily fixed by re-blocking the piece: Moisten the yarn and pin it back into shape on a blocking board or thick towel. Leave it in place until it's dried in that shape.