Thanks for looking into this topic and sorry for ambiguity, let me clarify shortly. At the moment - to my knowledge - the builder is enumerating and concatenating INO files in the given folder alphabetically, based on the file names. For this very reason, I have seen multi-file projects where INO file names are prepended with a_, b_, c_ etc, just to tell the builder the order, which is obviously not too elegant and also confusing. What I am suggesting is to have some sort of a project file, which. at its very basic implementation serves two purposes: 1 : list the files that is considered the part of the project, eg: what the builder should enumerate 2 : determine the order of these files (build-order) It is common to some other languages too, that a folder content and folder hierarchy within is considered to be the metadata used to organized files into namespaces, and also assuming that all files is also part of the project. While this approach looks straightforward for the first glance, but it is causing portability issues and file system dependency. Declaring the source files that contributes to the project has also the advantage to avoid situations where a file is copied to a project folder accidentally, and the builder - assuming the file belongs to the project - includes it, causing headaches later. So what I am suggesting instead is a simple (Json, xml, whatever) project file to fully decouple logical structure of a project from the file system structure. With Arduino IDE 2.0, the only viable option for a multi-file project is as follows: - separate the project to multiple header files (with the implementation, too), each spanning a single logical code domain, - add a dummy INO file which only has the includes *in the right order (isn't it the project file actually :-) ?)* Thank you.