Relative Twist of Yarns
Relative twist of yarns means comparing the twist in two yarns in a fair way, even when the yarns are of different thicknesses.
A thicker yarn and a finer yarn cannot be compared simply by saying both have the same number of turns per inch. For example, 12 turns per inch in a thick yarn will not give the same effect as 12 turns per inch in a fine yarn. This is because the fibres in a fine yarn lie on a smaller diameter, while the fibres in a thick yarn lie on a larger diameter. Therefore, the angle at which fibres spiral around the yarn surface becomes important.
The same relative twist is obtained when the angle of twist on the surface of the yarn is the same. In simple words, the fibres in both yarns are inclined at the same angle, even though one yarn is thicker and the other is finer. This gives a similar yarn character in terms of firmness, handle, strength, and appearance.
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Formula for Relative Twist
For similar yarns, the relative number of turns per inch is proportional to the square root of the yarn count.
This means that finer yarns need more turns per inch than coarser yarns to produce the same relative twist.
Example: 16s Yarn and 25s Yarn
For example, compare 16s yarn and 25s yarn.
The square root of 16 is 4.
The square root of 25 is 5.
So the relative twist required is in the ratio:
This means if 16s yarn has 12 turns per inch, then 25s yarn should have proportionately more twist.
12 ÷ 4 = 3
So each unit of relative twist equals 3 turns per inch.
For 25s yarn:
5 × 3 = 15
Therefore, if a 16s yarn has 12 turns per inch, a 25s yarn should have 15 turns per inch to have the same relative twist.
This does not mean that 25s yarn is “more twisted” in character. It means the finer yarn needs more actual turns per inch to create the same twist angle and similar yarn behaviour.
Why Relative Twist Is Useful
This concept is very useful in fabric manufacturing. Suppose a mill is producing the same type of cloth in different weights. A heavier version may use a coarser yarn, while a lighter version may use a finer yarn. To keep the cloth feel, appearance, and performance similar, the yarns should have the same relative twist.
For example, a coarse cotton fabric and a finer cotton fabric may both need a soft, smooth, balanced handle. The yarn counts may differ, but by adjusting the turns per inch according to the square root of the count, the manufacturer can maintain a similar yarn structure.
Limitation of the Rule
However, this rule works best when the yarns are of similar material, similar spinning method, and not extremely different in thickness. If one yarn is very coarse and the other is very fine, many other factors begin to affect the result, such as fibre length, fibre fineness, spinning system, yarn evenness, and intended fabric use.
In Simple Terms
In simple terms, relative twist helps maintain the same yarn character across different yarn counts. A finer yarn needs more turns per inch than a coarser yarn, but when the twist angle remains the same, both yarns behave in a similar way in the fabric.
