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Foundation is a modern framework favoring the mobile first approach. This means that the bare minimum code and assets are loaded first to aid page render speed. As with most modern frameworks, Foundation is fully responsive. Unlike most, it makes use of just one break point for structural change. The break point defines the viewport width at which the large grid is put into action, this happens at 768px.
The grid system itself actually has three parts; the desktop grid, the mobile grid and the block grid. As foundation is mobile first, let’s begin with the mobile grid.

Source : http://webdesign.tutsplus.com/tutorials/htmlcss-tutorials/foundation-for-beginners-the-grid-system/