The amount of twist in a yarn helps to define the style of yarn-
a yarn with a lot of air such as a woolen yarn will have much
less twist than a yarn with little air, like a worsted yarn. It
also affects the stretchiness of the yarn, strength, the halo of
the yarn, and many other attributes. Filling or weft yarns
usually have fewer twists per inch because strength is not as
important as with warp yarns, and highly twisted yarns are, in
general, stronger. Warp yarns have to be stronger so that they
can withstand the tension of the loom. The number of twists per
inch can, in plied yarns, be determined by counting the number
of bumps in one inch, and divide by the number of singles (the
strands plied together to make the yarn). If the picture to the
right, for example, was of an inch of 2 ply yarn, then the
number of twists per inch would be 6 divided by 2, or three, as
there are six bumps, and it is a two ply. While this method
works very well with plied yarns, singles don't have bumps to
count. One way to dete