Besides the quality and size of a sewing thread its twist is also of major significance to its sewability in the machine. The design characteristics of a sewing machine, such as hook or looper version (system) and the primary sewing direction, necessitate the use of a sewing thread with a certain twist direction.

An incorrect twist direction causes twist displacement in the sewing thread which leads to excessive thread wear and sewing problems. Twist displacement, and the accompanying unwinding of the thread, is caused when the thread is pulled through the needle eye in an unfavourable feeding direction (friction on the edge of the needle eye). In normal operation this can be observed during reverse feeding.

In connection with this the left needle of a two-needle machine also deserves mention, because the thread runs continuously in an unfavourable direction in this needle. On many automatic sewing systems the feeding directions are often unfavourable, too (mult-directional sewing). Great care must be taken to ensure that all thread-handling parts are in perfect condition and the machine is adjusted in the optimum way. In this case, special threads such as core-spun or bonded threads are most suitable.

Here is a rule of thumb for correct selection of the thread-twist direction:

all sewing machines that are threaded from left to right, viewed in feeding direction, require Z-twist threads. This is why most threads are available in Z-twist nowadays.