The Bootstrap grid system
Like Foundation, Bootstrap comes with a grid system that, by default, divides your working screen's real estate into 12 columns. Using classes, you specify how many columns wide you want every block on the screen to be. There are different classes for different sizes; this is Bootstrap's and Foundation's way to make responsive design easy and almost transparent.
Bootstrap and Foundation use different breakpoints. With Foundation, small means less than 640 px, medium is from 641 px to 1024 px, and large means 1025 px and higher. There are two optional XL and XXL sizes, like in T-shirts, with breakpoints of 1440 px and 1920 px. The last two are commented out; you need to activate them if you want to use them.
Bootstrap has four sizes: xs, sm, md, and lg. Just as two T-shirt manufacturers can have different sizes in small, the two frameworks have different sizes as well, as the breakpoints are 768 px, 992 px, and 1200 px.
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