I have been trying to figure how to actually count knots on a real rug. I looked at a diagram of the Persian knot and it looks to me that it covers two warps. If that is the case, should I divide the number of "bumps" by two? Sometimes it looks to me like there are two threads of the same color. Is the yarn doubled or is this 2 knots? I have a photo of the back of one carpet but I'm not sure how to attach it so it might not show up. I cut out a 1" by 1" square and put it over the back of the carpet. One row of knots got blacked out (because of the camera angle I guess). But if someone could clarify this it would be a bit help to me. Thanks to all.

Thank you. Your reply is very helpful. I did watch the video also. This is the first thing I have seen that actually explains and shows how to count knots. What I mean by "bump", is basically the raised portion that is visible from the back over each warp. So I conclude that each time the yarn goes over a warp is counted as a knot. The yarn in my particular case is double ply and that would account for 2 "bumps" each time (mostly) for each knot.