Grain Of Thread or Yarn
If you notice some of your stitches are clean and clear and others are fuzzy, you need to determine what is the grain of the canvas. We are talking about the little hairs that make the yarn look fuzzy.
Before threading a needle, check the grain. You want the fibers to travel with the thread and not against it. To do this, place a strand under your chin and stroke it. Turn the yarn around and repeat. One direction will tickle your chin more than the other. This is the fuzzy direction. The fibers should be pointing backwards. Thread your needle in the direction with the least amount of tickle.
Although cotton threads have a nap, it is not as prominent. Just thread up and stitch.
If you need two strands in the needle, never double over the yarn. Because one half of the yarn is against the grain, fuzzy stitches are created. Place two individual strands of thread in the needle’s eye.
You may think cotton, silk or rayon is not a problem. One strand doubled does not cover as well as two in the needle and your work will not be as smooth. With a doubled strand, you will not be able to rip out mistakes. The best rule to remember is, never double any thread.
Also see Good Canvas Coverage