OK. Mr. Bleu nudged me into. Let's tackle approach grafting next. Broadly speaking, approach grafting involves a shoot and an understock, both supported by their own roots. The cambium of the shoot and understock is exposed and the two are bound together. Over time, if the shoot and understock are compatible, they will graft and the shoot can be detached from it's supporting root system. Together with the detached scion or cleft graft and the thread graft we've already discussed, these three are the only grafts I've ever done. That said, there are many special use cases for each - grafting roots, grafting onto a thick trunk, etc. But everything I've ever done. Is one of these three grafts in various application.
So we undertand from our discussion that cleft grafts with detached scions are primarily used for junipers and pines whereas thread grafts are used mainly for broadleaf trees. The main advantages of an approach graft is that it has a higher success rate than cleft grafting and because the foliage doesn't have to be removed. That means we can use it not only for broadleaf trees, but pines and junipers as well. We can also use it for broadleaf trees such as Ficus Benjimina or boxwood that don't take very well to defoliation and thus may not respond well to thread grafting which requires that we remove all the leaves. It is also possible to graft fairly large branches in this way - certainly branches large enough to make thread grafting impractical. It is one of the most versatile of grafts and thus very important to master.
The primary disadvantage with approach grafting is the scarring. In thread grafting there is a simple entry and exit wound so that when the graft is severed there's only a small hole to callous over. With approach grafting,on the other hand, a strip of bark is removed in order for the scion to be attached. This can create callousing that can look unnatural, especially if the graft is done improperly. If all goes wrong and the graft fails, you only have two small holes to heal over with a thread graft. With an approach graft you are left with a nasty visible scar that can take years to heal. The damage may be permanent.