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Picks per inch/inch (or p.p.i.) is the number of weft threads per inch of woven fabric.
A pick is a single weft thread, hence the term. In general, the
higher the picks per inch, the finer the fabric is.
In weaving, the weft (sometimes "woof" which is nothing to do with a dogs bark) 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. The weft is threaded through the warp using a "shuttle", air jets or "rapier grippers". 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. You may also want to learn about ends per inch which is regarding the warp yarn. The warp is the lengthwise or longitudinal thread in a roll, while weft is the transverse thread.