Textile Guide: Yarn S/Z Twist Direction and Fabric Skew After Washing

1. First, Let’s Be Clear: Yarn Has a “Thread Direction”

Have you ever looked closely at a cotton yarn? If you rub it gently in your hand, you will notice that the fibers do not lie straight and parallel. Instead, they twist in one direction, almost like a rope. This twisting direction is called the twist direction.

If the yarn twist follows the middle stroke of the letter S, it is called S twist. If it follows the middle stroke of the letter Z, it is called Z twist. These two directions give yarn very different structural behavior.

Most single yarns are Z-twist yarns. This is because ring spinning machines usually twist yarn to the right by default, and this also matches common operating habits in production. S-twist yarns are less common and usually require custom production.

In simple terms, S twist and Z twist are like left-hand and right-hand screw threads. And the direction does not match, the yarn structure can loosen. If the directions work together, they can lock and balance each other.

2. How Single Yarn Twist Affects Fabric

A fabric is made by interlacing warp and weft yarns. Each yarn already has its own twist direction, and most of them are Z twist. When all yarns twist in the same direction, internal untwisting stress builds up inside the fabric.

During weaving and finishing, the fabric structure temporarily holds this stress in place. However, once the fabric is wash, the fibers absorb water and swell. At the same time, friction between yarns decreases, and the internal stress starts to release.

Each yarn then tries to rotate slightly in the opposite direction of its original twist. When many yarns do this at the same time, the whole fabric begins to skew diagonally. This is the hidden cause of fabric spirality, washing skew, and shape distortion.

A typical example is a cotton T-shirt. After several washes, the side seam may no longer stay vertical, and the hem may twist toward the front. Denim jeans can show the same problem: after washing, the pant legs may twist like a rope.

This is not always a cutting problem. Very often, it comes from unbalanced yarn twist.

3. The Solution: A Balanced Combination of S and Z Twist

To reduce fabric skew, the fabric needs torque balance. In other words, left-hand and right-hand twist forces should offset each other.

Method 1: Alternating S-Twist and Z-Twist Yarns

During weaving, S-twist and Z-twist yarns can be arrange alternately, such as one S-twist yarn followed by one Z-twist yarn. They can also be mix in a certain ratio.

The untwisting torque of S twist and Z twist acts in opposite directions, so the two forces can cancel each other out inside the fabric. High-end shirting fabrics and anti-skew denim often use this method.

The downside is cost. Warping and drawing-in become more complex, production efficiency drops, and the fabric becomes more expensive.

Method 2: Reverse Twist in Plied Yarn

Another common method is to use plied yarn. For example, two Z-twist single yarns can be twist together in the opposite S direction. This reverse twist helps offset part of the torque from the single yarns.

As a result, the final plied yarn has lower torque and better stability. Two-ply yarn can reach about 1.2 to 1.5 times the combined strength of single yarns, while three-ply yarn can reach about 1.5 to 1.7 times.

This method is widely used in sewing thread, premium shirting, and worsted wool fabrics. It makes weaving easier, and the yarn becomes rounder, smoother, and stronger.

Method 3: Heat Setting or Mechanical Preshrinking

For thermoplastic fibers such as polyester, heat setting can reduce internal stress after weaving.

For natural fibers such as cotton, mechanical preshrinking, such as Sanforizing, and skew correction can help control distortion during finishing.

In denim finishing, mills often use a skewing process. Left-hand twill fabric is pulled in one direction, while right-hand twill fabric is pulled in the opposite direction. This compensates for the stress caused during weaving.

However, these methods can only reduce skew. They cannot remove the root cause completely.

4. How Fabric Skew Is Measured

In the textile industry, washing skew is usually measured by skewness rate or twist angle.

China’s GB/T 23319 standard, Textiles — Determination of Distortion After Washing, provides test methods. However, it does not set one universal pass/fail limit. Different products follow different product standards.

For knitted T-shirts under FZ/T 73008-2002, first-grade products require a washing skewness rate of no more than 6.0%.

For premium suits, shirts, and formalwear, the usual requirement is around 2% to 3%.

For regular crewneck shirts and polo shirts, the common requirement is around 4% to 5%.

For trousers and sleeves, the usual limit is around 3% to 4%.

Internationally, ISO 7769 commonly uses a washing twist angle limit of no more than 10 degrees.

GB/T 21295-2024, Technical Requirements for Physical and Chemical Performance of Garments, came into effect on October 1, 2024, and updated related requirements for washing skewness. When buying garments, you can check whether the product follows this standard.

Fabric skew affects more than appearance. It can cause side seams to shift, pockets to move out of position, and zippers to look crooked. Many “bad fit” complaints may actually come from unbalanced yarn twist, not poor cutting.

5. Other Effects of S/Z Twist Direction

Twist direction affects more than washing skew. It also influences fabric luster, hand feel, pilling, and dyeing.

Fabric Luster

S-twist and Z-twist yarns reflect light in different directions. When warp and weft yarns use different twist directions, such as Z twist in the warp and S twist in the weft, the fiber angles can align better. This creates a more consistent reflection and improves fabric luster.

Tartan fabrics often use this effect to create a unique shine.

Hand Feel and Pilling Resistance

Reverse-twisted plied yarn, such as Z-twist singles twisted together with S twist, can make the yarn surface smoother. It reduces hairiness, improves hand feel, and helps resist pilling better than single yarn.

Dyeing Performance

High-twist yarns usually have slightly lower dye penetration. S-twist and Z-twist yarns may also show small differences in dye uptake because of their structural symmetry.

For premium fabrics, mills adjust the dyeing process to keep color difference within an acceptable range.

6. Buying Tips: How to Avoid Fabric Skew

1. Do a wet-twist test on white shirts. Soak one sleeve in warm water for about 10 minutes, squeeze out the water, then lay it flat to dry. Check whether the side seam twists obviously. A high-quality shirt should keep the side seam mostly vertical after washing.
2. Choose two-ply or high-count fabrics when possible. Two-ply yarns, such as 80s/2, usually use Z-twist single yarns and S-twist plying. They resist skew better than single yarns such as 80s/1. Labels that mention “two-ply” or “plied yarn” are usually a good sign.
3. For denim, look for anti-skew or preshrunk treatment. Many raw denim brands mention “anti-skew,” “Sanforized,” or “alternating S/Z twist” on the label. If there is no such information, the jeans may twist after the first wash.
4. Check whether the fabric has a subtle herringbone-like texture. If you can see tiny alternating left and right diagonal lines, the fabric may use S/Z alternation, which improves anti-skew performance. This is easier to see on checks or stripes than on solid fabrics.
5. Understand the price difference. S/Z alternating weaving lowers efficiency and increases cost. Two-ply yarn can cost 30% to 50% more than single yarn. Shirts or jeans at very low prices usually do not use these processes.
6. Ironing can temporarily improve skew. If a garment has already twisted, you can stretch and iron it diagonally while it is damp. This may improve the shape for a short time, but the problem can return after the next wash.

7. Common Misunderstandings About Yarn Twist

“All fabrics twist after washing. It is normal.”

Slight distortion within about 3% can be acceptable for ordinary garments. However, visible twisting beyond the product standard usually means the process is not well controlled.

“Fabric skew is the same as shrinkage.”

No. Shrinkage means the overall size becomes smaller. Skew means the shape rotates or twists. Shrinkage can be improved by preshrinking, but skew needs twist balance or skew compensation.

“S twist is better than Z twist.”

Not true. Neither direction is automatically better. Z twist is common for single yarn because of industry practice. S twist usually needs custom production. High-end fabrics often use both directions together.

“You can check twist direction by rubbing the yarn.”

Yes. Pull out one yarn and observe the fiber direction. If it follows the middle stroke of the letter S, it is S twist. If it follows the middle stroke of the letter Z, it is Z twist. Most ordinary cotton yarns are Z twist.

Conclusion

S twist and Z twist are a hidden balancing technique in textile engineering.

If all yarns in a fabric twist in one direction, the fabric may distort after washing, almost like a towel being wrung. When S twist and Z twist work together, or when plied yarn uses reverse twisting, the internal stress can be balanced before it becomes a visible problem.

So, when your favorite shirt twists after several washes, it may not be your washing method. The real reason may be that the fabric did not have the right twist balance inside.

Learn to check labels, ask about yarn construction, and do a simple wet-twist test. A well-balanced fabric will keep its shape longer and look better after repeated wear and washing.