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When and How to Effectively Use Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development built at Twitter by @mdo and @fat. It contains CSS - and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components. It uses a 12-column responsive grid system which will re-arrange depending on the screen size. You do not need to know CSS extensively or at all to be able to use Bootstrap, all that's needed is the Framework.