Thread twist and sewing

What effect does the scan direction of polyester threads have on sewing quality.

Roman Schmock - October 18, 2023

Effect of thread scan direction on sewing

I received an interesting question and it would be a shame not to share it.

“What effect does the scan direction of polyester threads have on sewing on a sewing machine.”


A bit of theory

Note: I have already covered the subject of threads in more detail in previous articles, namely here: Threads and here: Marking of threads . However, in these articles I did not describe the direction of the thread scans, so we will clarify this now.

  • Scanning is the process by which fibers are joined together.
  • The twist indicates the amount of turns that are put into the thread during the manufacturing process.
  • The scanning direction can be either left or right.

Thread scanning direction

The direction and number of turns of both the input fibers and the resulting threads are key to the character of the woven yarn. The direction of the twist is obvious when looking at the thread held vertically. If the spiral of the twist runs in the same direction as the middle part of the letter “Z”, it is a right-hand thread. If the thread is turned away from the center, it looks more like the letter “S”.

When scanning with the same direction of twist for both presented threads, a very strong yarn is produced. On the contrary, when spinning in the opposite direction during scanning, a softer yarn is produced.

As the number of twists increases (up to a certain limit), the strength of the yarn increases.

Taken over and more about Thread Scanning can be found here: Scanning | Wikipedia


The amount of twist affects the strength and appearance of the thread. A higher amount of twist can increase the strength of the thread, but it can also make it harder and less pleasant to the touch. Conversely, a lower amount of twist leads to finer and softer threads, but with lower strength.

Effect of scanning direction on sewing quality

ATTENTION - For simplicity, I will deal only with polyester threads suitable for ordinary sewing on a home sewing machine.

Sewing machine theory

From a technical point of view, the direction of the thread twist has an effect on the sewing quality, depending on the design of the sewing machine. Most sewing machines are designed to use right-hand “Z” twist threads, meaning that the thread turns in a clockwise direction.

From a technical point of view, more attention is paid to the lower thread. The direction of twist of the lower thread depends on the design of the sewing machine and the direction of movement of the hook. If the bobbin thread has a reverse twist to the direction of movement of the hook, the thread may unravel and catch poorly on the needle, causing uneven sewing.

Our experience

In our practical experience, we did not focus directly on the twist direction of the threads, rather we focused on the material and strength of the threads. If you are using regular polyester threads of adequate strength for both upper and lower threads, you should not encounter any problems.

I hope this brief answer provided you with the information you required. If you have any further questions, please do not hesitate to contact us again.