A construction of yarn in which the lengthwise and vertical yarns are carried over and under the weft. This type of warn is comprised of more twist than weft yarns and deal with more stress in the weaving process. In the world of denim, warp runs parallel to the selvage.
Warp threads, also known as surface threads or ends, are stretched vertically on the loom and are therefore made of stronger, coarser fibers than the weft threads. The term comes from the Old Norse word “varp”, meaning “the cast of a net”. Similarly, warp threads work like a net to hold the weft threads in place so the fabric does not unravel. With jeans, the warp threads are the ones that are dyed indigo and make up the majority of the thread one sees on the right side of denim.
Here is an example of warp yarn: