With AVR Studio 5, the concept of "solution" is introduced. The solution is a container that may contain several projects. A project cannot exist outside a solution. If you try to open a project file ( .cproj or .asmproj extension) a solution will be created for you. This allow you to keep for example a bootloader project, and several application projects in the same solution. In practice the Solution is stored as an .atsln file. In general, projects that are added to the solution are placed in a separate folder inside the folder that the .atsln file recides in.