The vast majority of fruit trees consist of two parts, the scion or fruiting variety, and the rootstock. They are joined at the graft-union, which on a young tree can be seen as a kink in the stem, near the ground.
In some cases a second piece of grafting material is used, with the result that the tree has two graft unions, and is therefore "double-worked". This extra work adds cost to the production process in the Nursery, but is done for various reasons.
The most common kind of double-working involves the insertion of a short piece of graft material just above the main graft. This grafting is usually carried out in the Nursery at the same time as the main graft, and such trees have a characteristic double kink in the lower part of the stem.
There are two main reasons for using interstems. Firstly, they can be used to achieve a combination of rootstock characteristics. A common interstem is the MM111/M9 combination - a short piece of M9 is grafted on to the MM111 rootstock, and then the scion variety is grafted on top of the M9. The result is a tree with the benefits of both rootstocks - in other words free-standing and drought-tolerant thanks to the MM111, but precocious and not too big, thanks to the M9. Although these dual-rootstock interstem trees fulfill a useful purpose, we expect production to fall away as new rootstocks become available which achieve the same result without the expense of double-working.
Secondly, interstems can also be used to deal with incompatibility between the rootstock and the scion. This is particularly useful in pears, where some varieties are not compatible with the Quince species rootstocks that are the only way to achieve a dwarf pear tree. In this case the interstem does not restrict the size of the tree, but acts as a buffer between the scion and the rootstock.
Stem-builders are another variation on the double-worked tree. Whereas the two grafts on an interstem tree are usually only a 10m-20cm apart, with a stem-builder tree the second graft is likely to be 1m-2m above the primary graft. In effect stem-builders are therefore "top-worked".
A stem-builder tree is grown in two stages. Firstly a vigorous scion variety - often Bulmer's Norman in the case of apple trees - is grafted on a vigorous or semi-vigorous rootstock. In the first year in the Nursery this grows very rapidly and achieves a thick girth. Then in the second year the required scion variety is grafted on the top of the resulting tree, at which point the stem-builder is effectively a very long interstem. However its purpose is neither to help with compatibility or control size, it is just to create a more impressive-looking young tree.
From the Nursery point of view it also means that there is the opportunity to delay committing to particular scions for a year, so a two-year tree can be offered to the customer with only one year's notice.
Stem-builders are almost exclusively used for producing standard trees on vigorous rootstocks for large-scale traditional orchard regeneration projects. For most other purposes a one-year tree or a regular two-year tree on a vigorous rootstock will be more suitable.