In weight per unit length or length per unit weight the fiber Fineness can be expressed. According to Textile Institute the fineness of cotton, silk and manufactured fibers are usually expressed in term of average linear density. Every single fiber has variable cross-section along its length. And every single fiber can be varies in cross-section shape from fiber to fiber.
Mass = volume × density
=Cross-sectional area× length × density
For a known length or unit length, Mass is proportional to cross-section.
To measuring a known length of fiber by using this formula is called linear density and this expressed in weight per unit length.
Importance of Fineness:
1.     Lower irregularity of yarn.
2.     Uniformity of count.
3.     Uniformity of strength.
4.     Less twist.
5.     Good appearance.
6.     Less neps.