Yarn count systems
To measure yarns, it is not possible to measure their diameter because the crosssection is so easily deformed, so we have to resort to distinguishing them by yarn count.
Yarn counting is based on knowing two measurements:
 the length (L)
 the weight (W).
By relating these two measurement we get values called:
 Counts (T) in the direct counting system;
 Numbers (N) in the indirect counting system.
Direct method
The direct method, called the fixed length, variable weight method, is given by the ratio
of the weight divided by the length (T=W/L).
This method is commonly used for all yarns made with continuous fibres
(silk, rayon, nylon etc.)

The Tex count (Tex) indicates the weight in grams of 1,000 metres of yarn.
For example, Tex 1 means that 1,000 metres weighs 1 gram. 
This counting method is called universal and ought to replace all
other count systems, especially the indirect system (N). 
The Decitex (Dtex) count indicates the weight in grams of 10,000 metres of yarn or filament.
This method is used for filaments or multifilament yarns.
For example, Dtex 1 means that 10,000 metres weighs 1 gram (filament). 
The Denier (Den) count indicates the weight in grams of 9,000 metres of yarn or filament.
It is used for filaments, for multifilament yarns and for silk.
For example, Den 1 means that 9,000 metres weighs 1 gram (filament).  With the direct method, the number indicating the count is higher the larger the yarn (or filament), the count being expressed as a function of the variable weight compared with a fixed length.
Indirect method
The Indirect method, also called the fixed weight, variable length method,
is given by the ratio between the length and the weight (N=L/W). The count by this
method (N) is commonly used for all yarns made with discontinuous fibres (wool, cotton, rayon staple etc.). The most commonlyused indirect counts are:
 metre and kilogram count;
 English cotton count;
 Prato count.
There are also the English carded wool count, the English combed wool count,
and the English flax and hemp count.

The metre number (Nm) indicates how many metres of yarn there are in 1 gram.
For example, Nm 1 means 1 metre of yarn in 1 gram. 
The kilogram number (Nkgm) indicates how many metres of yarn there are in 1000 grams.
For example, Nkgm 1,000 means 1,000 metres of filament in 1,000 grams.
This count is used especially in the Prato area and also in the Biella area
for large yarns. For example, Nm 7 is also called Nkgm 7000 or simply 7000. 
The English cotton count (Ne, Nec or Ne cotton) indicates how many 768 metre
hanks (equivalent to 840 yards) there are in 454 grams (equivalent to 1 pound) of yarn.
For example, Ne 1 means that one 768 metre hank of yarn weighs 454 grams.
This count is used for cotton and is also in use in Italy. 
The Prato number (Np) indicates how many 583 metre hanks (equivalent to 1000 Tuscan
"arms") there are in 339.5 grams (equivalent to one Tuscan pound) of yarn.
For example, Np 1 means that one 583 metre hank of yarn weighs 339.5 grams.
The Prato number is divided, in the decimal part, into quarters (¼, ½, ¾) and is used
in the Prato district for carded yarns only. It follows from the above,
the number being a function of the variable length compared with the fixed weight,
that the higher the number the finer the yarn.