What softness in clothes actually comes from
Short answer: Softness comes from the thickness and surface of the fibres, from how the yarn is spun, and from the knit. It is not one property but several things working together.
The thickness of the fibre
Thin fibres bend more easily and therefore feel softer against the skin than thick ones. That is the single most important factor.
The surface
A smooth fibre glides against the skin. A rough fibre such as wool has scales that can scratch. See why wool scratches.
The yarn
An evenly spun yarn has fewer fibre ends sticking out and feels smoother. See quality t-shirt versus cheap t-shirt.
The knit
A loose knit feels softer than a tight one in the same yarn, because the fabric gives way more.
Finishing
Some fabrics are made soft with a treatment that washes out after a few months. That is why certain t-shirts feel different after six months.
Where you notice it
Loungewear and nightwear are where softness matters most. See bamboo loungewear for women.
See also
More on the subject in why bamboo viscose feels so soft.
Frequently asked questions
Why do some t-shirts turn less soft after six months?
Because a finishing treatment has washed out. Softness from fibre and knit remains.
Are thin clothes always softer?
Often, because thin fibres bend more easily. But the knit also matters a lot.
What makes wool scratchy?
The fibre surface has scales that can irritate the skin.
