Its a few things. Mainly a "regular sized" tree (or variety of a regular tree) is grafted onto a dwarfing rootstock. It goes beyond that but I know mainly of dwarf rootstock with normal varieties scions grafted onto dwarf rootstock. I think a very few amount of trees are technically "genetic" dwarfs from seed. The pros will know more.

interesting. Thank you

I agree with blazeaglory. I have been reading about dwarf plants for many years and most of them are dwarfed using certain root stocks. One nursery used a three piece stem with a dwarfing piece between the rootstock and the top. I don't think I have seen that method used on citrus. I have read the rootstocks used for oranges but I don'remember what they are.

Flying dragon and sour orange?, I think, are two dwarfing rootstocks but there are more (I think).

Sour orange is not a dwarfing rootstock, it is a an older, standard rootstock which tends to be susceptible to Tristeza virus (so it is not used commercially in California much anymore). You may hear references to "heirloom" oranges, and this is frequently referring to old orange orchards on Sour Orange. Some old-timers feel this produces a very sweet orange. Trifoliate varieties (which includes Flying Dragon and Swingle), Citrange varieties (such as Troyer and Carrizo), as well as Cuban Shaddock (which is a true dwarfing rootstock, and what Four Winds uses very successfully) are the main types of semi-dwarfing and dwarfing rootstocks you see most commonly in the commercial market. There are others but these are the main varieties.

Patty S.

Here is a link that might be useful: UC Riverside Citrus Variety Collection: Citrus Rootstocks, Their Characters and Reactions

I wonder if sour orange really does produce a sweeter orange? Its funny that its called "sour" but produces a "sweet" orange.