A model of subobjects and subobject selection gives us a concise expression of key semantic relationships in a variety of inheritance-based languages. Subobjects and their selection have been difficult to reason about explicitly because they are not explicit in the languages that support them. The goal of this paper is to present a relatively simple calculus to describe subobjects and subobject selection explicitly. Rather than present any deep theorems here, we develop a general calculus that can be used to explore the design of inheritance systems.