Extra warp, also known as supplementary warp, is where additional warp ends are inserted and used to create a decorative pattern on top of the ground cloth.
The ground cloth is usually kept simple, such as a plain or twill weave, to enhance the extra warp. This extra warp is woven on separate shafts to the ground cloth so that it can be woven independently to create the decorative pattern without interrupting the ground cloth weave.
The example below shows an extra warp design using 8 shafts. Shafts 1-4 are for the ground cloth while the extra warp is on shafts 5-8. The black extra warp ends are inserted at regular intervals along the width of the warp.
The extra warp ends have been alternated with the ground cloth ends so that they do not interfere with the ground cloth weave. If the extra warp ends were all put in next to each other the ground cloth may end up having gaps.
The white ground cloth ends are put through the reed at a rate of 30 ends per inch. The extra warp threads are put in as additional threads so the warp is actually threaded at a higher density at these point. This is so the ground cloth remains consistent throughout. It needs to be able to weave a plain weave when the extra warp ends are not being woven in.
As you can see there are two woven structures above. In structure 1 the extra warp threads are floating over the surface of the cloth whereas in structure 2 the threads are woven in to the cloth. I have decided to cut the floating threads away. This will leave the black extra warp ends where they have been woven into the ground cloth creating rectangles. Structure 2 can be woven for fewer picks to create small squares or more picks to create longer rectangles.
The resulting sample is below.