An interstem is a piece of stem material grafted between the rootstock
and the desired variety.
 
 
Interstems are used for two main rreasons:
 

1. To reduce vigour and control the growth of the cultivated
   variety - the apple rootstocks M9, B9 and M26 are commonly
   used as interstems for this purpose.
    

2. As a compatibility aid, to assist in grafting two incompatible varieties Beurre Hardy is used as a compatibility aid between
   quince rootstock and an incompatible pear scion

Usual nursery culture is to grow the rootstock in the ground for one year,
then graft the scion and interstem onto the rootstock in the ground (double graft).
You can also double graft straight away using the whip and tongue method
or use a modified T bud of the interstem and the variety