Sandy has lately been preaching a new and simple way of looking at agility. I haven't had much time to think about it yet, but when she talks about it in class it makes sense. Although now that I'm listing them I don't see anything about rear crosses, so maybe that should be in there too. Although I think that's more a style of handling that an element of a course.
The theory is that you can break down every agility course into the following elements:
Pulls (I can't remember if this was one. I have to double check with Sandy)
If you train your dog to know how to handle these elements (and yourself to handle your dog through them consistently) then your dog can do any course out there. The trick is identifying the element within the course correctly. Quite often people won't identify them correctly.
So for example in this course, 2-3 is a straight line and should be handled as such.
But in this course, after just rotating the broad jump slightly, this turns into a threadle because if your dog is trained to know 270's their natural path is to take them there. If you don't handle it as a threadle and they come into the jump then they don't understand how to do 270's independently, which means that at that point you have to be there at jump #3 to push them around the backside. In doing this you would lose an advantage of being able to leave the dog to handle that on his own and get into position for jump #4. Independent 270s can be a wonderful thing!