This might sound a bit dumb.

I always had this impression that web.config should store all settings which are suspect to change post-build and setting.settings should have the one which may change pre-build.

but I have seen projects which had like connection string in setting.settings. Connection Strings should always been in web.config, shouldnt it?

I am interested in a design perspective answer.

Just a bit of background: My current scenario is that I am developing a web application with all the three tiers abstracted in three separate visual studio projects thus every tier has its own .settings and .config file.