Hi Steve,
The 'M' series of rootstocks derives from the East Malling Research
Station in
England. The original Malling-Merton 106 introduced by them in 1969 has
since been
mostly replaced with a more virus-free clone called EMLA 106. It results
in a tree
about 65% the size of a standard size. There are a whole series of these
rootstocks
including EMLA 7(55-60% dwarfing), EMLA 9 (30-40% dwarfing), EMLA 26
(40%),
EMLA 27 (15-20% dwarfing, one of the smallest trees in this series), and
EMLA 111 (75% and similar to the 106, but more vigorous). There are other
rootstocks with this
series naming like Geneva, which comes from a Cornell project. There are
also more
Malling versions that I have not mentioned. You can find a more complete
list in the
book Fruit, Berry, and Nut Inventory available from the Seed Saver
Exchange or
possibly a Botanic Library.

Sherwin D.

Steve Newport wrote:

Hi all,

I have a Sunset Apple tree that we have inherited with our new house.
Its about five feet tall which, I understand from a few websites, is
about the size they supply plants to start with. I am also told that
pruned it should easily maintain a height of 6-8 feet (ideal for me in
its position) or 12-15 if left untouched.

Then it mentions that the tree is on a rootstock of MM-106

I understand what they mean by rootstock but exactly what is MM-106
and where did the numbering system come from?