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Ends per inch (EPI or e.p.i.) is the number of warp threads per inch of woven fabric. In general, the higher the ends per inch, the finer the fabric.
- In weaving, the weft (sometimes woof) is the thread or yarn which is drawn through, inserted over-and-under, the lengthwise warp yarns that are held in tension on a frame or loom to create cloth. Warp is the lengthwise or longitudinal thread in a roll, while weft is the transverse thread. A single thread of the weft, crossing the warp, is called a pick. Terms do vary (for instance, in North America, the weft is sometimes referred to as the fill or the filling yarn). Each individual warp thread in a fabric is called a warp end or end.
Ends per inch is very commonly used by weavers who must use the number of ends per inch in order to pick the right reed to weave with. The number of ends per inch varies on the pattern to be woven and the thickness of the thread.
Plain weaves generally use half the number of warps per inch for the number of ends per inch, whereas denser weaves like a twill weave will use a higher ratio like two thirds of the number of warps per inch.
Plain weave (also called tabby weave, linen weave or taffeta weave) is the most basic of three fundamental types of textile weaves (along with satin weave and twill). It is strong and hard-wearing, used for fashion and furnishing fabrics. In plain weave, the warp and weft are aligned so they form a simple criss-cross pattern.
Finer threads require more threads per inch than thick ones, and thus result in a higher number of ends per inch.
The number of ends per inch in a piece of woven cloth varies
depending on the stage of manufacture. Before the cloth is woven, the
warp has a certain number of ends per inch, which is directly related to
the size reed being used. After weaving the number of ends per inch will
increase, and it will increase again after being washed. This increase
in the number of ends per inch (and picks per inch) and shrinkage in the
size of the fabric is known as the take-up. The take-up is dependent on
many factors, including the material and how tightly the cloth is woven.
Tightly woven fabric shrinks more (and thus the number of ends per inch
increases more) than loosely woven fabric, as do more elastic yarns and
fibers.
You may also want to learn about picks
per inch which is regarding he weft yarn. Reminder, the Ends
Per Inch is regarding the "warp" yarn.