What is Specific Length?
Specific length is the length per unit mass, or a measure of the linear weight of some one-dimensional substance such as a wire or thread. Its practical application is in measuring the weight of threads and yarns in the textile industry.
The SI unit of specific length is m/kg, but as might be expected in a specialized industry such as textiles, there are many industry-specific units of measurement in use. Some of these are defined in terms of the length of thread needed for a given weight, and others use the reciprocal units of mass per unit length - a sort of linear density (e.g. Tex which is the SI unit).
Measurements include the metric yarns number (Nm) which is the length in metres per gram of mass, and the cotton count which is the number of pieces of 840 yard threads needed to make up a mass of one pound.