So far in this book we have classified rocks into igneous, sedimentary, and metamorphic, and we have discussed the origin of magmas and igneous rocks using only a few common rock names, such as basalt, andesite, and granite. In this chapter, we consider the classification of igneous rocks in more detail, and we see, for example, that some rock types commonly occur together, whereas others are never associated. In Chapter 8, we learned about igneous rock-forming processes – why rocks melt, where they melt, and how they melt. We saw that the melting process controls the composition of the liquid, which determines the composition of magmas that ascend into the crust. We also learned about the physical properties of magma and of processes that can change its composition and, hence, the spectrum of igneous rocks it can form. We are in a position to use this knowledge, first, to describe where igneous rocks form; second, to learn how igneous rocks are classified; and finally, to relate igneous rocks to plate tectonics.