how to put two controllers/methods on one url

I'm a basecamp user. At basecamp URl such as, both projects and milestones are controllers (are they?), 488470 is project id and new is method of milestones. Usually, you get http://hostname/controller/method/otherinfo, but how they put their url together and let rails now how to interprete the URL? via route? Can any body gives some details?

Thanks a lot!