## Project Hierarchies

So far we've only considered examples with one project —a. with one user-written Boost.Jam file, `Jamroot`). A typical large codebase would be composed of many projects organized into a tree. The top of the tree is called the project root. Every subproject is defined by a file called `Jamfile` in a descendant directory of the project root. The parent project of a subproject is defined by the nearest `Jamfile` or `Jamroot` file in an ancestor directory. For example, in the following directory layout:

    top/
      |
      +-- Jamroot
      |
      +-- app/
      |    |
      |    +-- Jamfile
      |    `-- app.cpp
      |
      `-- util/
           |
           +-- foo/
           .    |
           .    +-- Jamfile
           .    `-- bar.cpp

the project root is `top/`. The projects in `top/app/` and `top/util/foo/` are immediate children of the root project.

| Note
When we refer to a “Jamfile,” set in normal type, we mean a file called either `Jamfile` or `Jamroot`. When we need to be more specific, the filename will be set as “`Jamfile`” or “`Jamroot`.”

Projects inherit all attributes (such as requirements) from their parents. Inherited requirements are combined with any requirements specified by the subproject. For example, if `top/Jamroot` has

    <include>/home/ghost/local

in its requirements, then all of its subprojects will have it in their requirements, too. Of course, any project can add include paths to those specified by its parents. [4] More details can be found in the section called “Projects”.

Invoking bjam without explicitly specifying any targets on the command line builds the project rooted in the current directory. Building a project does not automatically cause its subprojects to be built unless the parent project's Jamfile explicitly requests it. In our example, `top/Jamroot` might contain:

    build-project app ;

which would cause the project in `top/app/` to be built whenever the project in `top/` is built. However, targets in `top/util/foo/` will be built only if they are needed by targets in `top/` or `top/app/`.

[4] Many features will be overridden, rather than added-to, in subprojects. See the section called “Feature Attributes” for more information