Woof: the sideways threads that make cloth

In everyday speech, woof is the sound a dog makes. But in textile mills and weaving sheds, woof names one half of the two-thread system that forms woven cloth. Specifically, woof is the fill yarn, the thread that runs horizontally across the loom, passing over and under the lengthwise warp threads to lock them together and build the body of the fabric.

The woof thread works crosswise to the warp. While warp threads run the length of the loom and remain under tension throughout weaving, woof is inserted one row at a time by a shuttle, a needle, or an air jet, depending on the loom type. Each pass of woof is called a pick. The interlacing of warp and woof, and their density, determines the weave structure, the hand of the fabric, and its strength in both directions.

The word woof (also spelled weft, the older and still common term) comes from the Old English word meaning to weave. Warp and woof are a paired vocabulary, as necessary as length and width. In some trades, weft is preferred, especially in more formal or technical contexts, but woof remains standard in mills and among weavers in English-speaking countries.

Because woof is inserted fresh with each pick, it can be switched out more easily than warp. This flexibility lets weavers change the color, texture, or yarn type of the woof to create stripes, patterns, or special effects while keeping the same warp in place. A single warp can yield many different fabrics simply by varying the woof.