Responsive web design is a collection of techniques for building websites that work on multiple screen sizes. In these lessons, we’re going to use the foundational principles of responsive design as a framework for thinking about page layout. By the end, you should have a better understanding of how to approach common decisions in responsive design.
In general, responsive design consists of fluid grids, fluid images, and media queries. By this point, you should have already done some responsive design, but understanding the origins and foundational principles will help you step into more advanced concepts.

TreeHouse Responsive Layouts