This document discusses the key building blocks that enable Scala to be a modular and growable language. It outlines seven simple parts of the Scala language itself that can be combined in flexible ways, including expressions, scopes, patterns, recursion, function values, collections, and vars. It also lists seven features of Scala's type system that support modular programming, such as static typing, objects, classes, traits, abstract types, parameterized types, and implicit parameters. The document argues that this combination of simple language parts and powerful type features allows Scala programs and libraries to be composed modularly and to grow in complexity in a manageable way.