I got this information from an old pamphlet on smocking and I've never run into it anywhere else. Using it really improved the look of my cross stitch. Every thread has a twist to it. After separating the 6 strands of embroidery floss, be sure to keep them running in the same direction and let them dangle their twist out far enough away from each other that they don't tangle. Wet or wax each strand and put them together with the same grain direction. After a little practice running your fingers down the thread one direction then the other, you will be able feel that there is a smoother direction and a more textured one. Make sure you thread the end that will have the thread going in the "being smoothed" direction. It might sound tedious, but the process itself works to smooth and align the threads so the stitches look full and neat.
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Twisting thread by: Anonymous
I find that by separating the individual strands before threading the needle usually stops the threads from twisting unless a very large area needs to be covered. I was really pleased when I found this method.
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Twisted Threads by: Alix
The previous posts offer the best solutions, but many stitchers also use a laying tool to lay their threads. After you come up through the fabric you stroke the thread until it is untangled and then put the needle down while still stroking. You may see this technique on Shay Pendray's needlework studio that is aired on Public Television. It works wonders when using multiple plys of thread. Amy Bunger's needlework videos may also have it demonstrated on them.
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twisting thread trouble by: Valerie Kalyani
There are two ways to deal with this. The most common one is to let your thread and needle hang freely every few stitches and untwist itself. The other (which I prefer) takes a bit of practice. As you sew, when you take hold of your needle to pull to the front of your work, twist it a teeny bit, about a quarter-turn in the opposite direction it wants to twist. You will have to practice a while but if you master this simple technique you will not have twisting thread any more and will only rarely have have to dangle it to untwist.