Most responsive design is talked about as a matter of layout. A grid collapses. A column reflows. A menu hides. The good responsive design we have made was almost never about layout — it was about line length.
A line of body text wants to be between forty-five and seventy-five characters. On a phone, that is roughly one column at twelve points. On a poster, it is one column at sixty. Our grids are built to hold that range, not to break it.
A grid that breathes is one that lets the same essay set well from a 320-pixel screen to a billboard. It is hard. It requires you to set type by hand at multiple sizes — and then to write a typography system that the developers can implement without re-doing your work. We have written this guide so we never have to give the same talk again.