The Northern Pin Oak or Hill's Oak (Quercus ellipsoidalis) is an oak in the red oak section Quercus sect. Lobatae. It is mainly native to the northern midwest United States, and also in the southeast and southwest of Ontario, Canada. It occurs on dry, sandy, usually acidic soils. Although the name suggests an alliance to the Pin Oak Q. palustris, it has traditionally been thought to be closely related to the Scarlet Oak Q. coccinea, and was in fact included in that species by many botanists. However, recent work suggests that Hill's oak is more closely related to Black Oak Q. velutina and that there may be ongoing gene flow between those species (Hipp and Weber 2008). The morphological similarity between Q. ellipsoidalis and Q. coccinea remains a source of confusion, especially in northwestern Indiana and southern Cook County, Illinois.