let you construct complex web designs for multiple sized screens all at once using a grid system. It is a visual design tool that helps you create your layout first for three sized devices (screen, tablet, and smartphone), and the design is built to be responsive, so that you create the design on one page in three ways and the layout changes depending upon the size of the device viewing it. Designers who prefer to use grid systems (such as 960 Grid System) for design will find it very easy to get used to