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Bootstrap-3 - Interview Questions
What is Bootstrap 3 Rows & Columns ?
Inside the container, the .row class is used to contain the grid columns. Rows should always be placed inside of a container (.container or .container-fluid) to ensure 

proper spacing (between page content and the edge of the browser). 

The Bootstrap .row uses negative margins (-15px) to counteract the padding (15px) of the container. The end result is no visual spacing (margin or padding) on the 

sides of the row within the container. There are several reasons why the “negative margin” approach is used in Bootstrap.