I'm showing you a fragment of a carpet because it's the easiest way to see the structure, especially if, like this one, it is worn in places. So here the pile has actually worn away completely. And it enables us to see the pale coloured warp threads and the dark brown weft threads which form the structure of the carpet.
Some people, when they look at a carpet like this, think that the knots have been added once the warp and the weft have been woven. But it isn't true. The knots are actually tied as the carpet is being woven.
The weft in this carpet is dark brown wool, which you can see here. It isn't as tightly spun as the warp threads because it doesn't have to bear as much of the weight of the carpet. After every row of knots - and here is a row of red knots - several threads of dark brown wool have been inserted. They've been interlaced with the warp threads - going over one warp thread and then under the subsequent one. They form a plain weave structure which is what's holding the carpet together. If you actually pulled out every single knot in a carpet, you would end up with a piece of plain woven fabric that is still stable, still very secure. All the pile does is add colour and pattern and a certain degree of thickness so the fabric becomes more durable because if you are puting it on the floor, it has to be hard wearing.