Unlike apples no viable commercially available rootstocks that consistently control tree size exist yet. The most common rootstock used is seedling. You may occasionally find pears grafted onto hybrid rootstocks developed from crossing Old Home and Farmingdale varieties. These selections were developed because of their moderate resistance to fire blight. The nursery catalogs will list them as OHxF. Pears are also sometimes propagated on selections of Quince (Cydonia oblonga). However, some varieties like Bartlett and Bosc are not compatible with Quince. Asian pears are, by nature, weak-growing trees and, therefore, should be grafted onto more vigorous rootstocks such as seedling.