Bootstrap - col-[xs/sm/md/lg]

I always get confused if we need to specify col-xs, col-sm, col-md and col-lg classes for a column in bootstrap. If we specify col-xs it gets applied to xs, sm, md and lg unless we have different value for sm, md and lg. 

http://getbootstrap.com/css/ 

Introduction:

• Grid classes apply to devices with screen widths greater than or equal to the breakpoint sizes, and override grid classes targeted at smaller devices. Therefore, e.g. applying any .col-md-* class to an element will not only affect its styling on medium devices but also on large devices if a .col-lg-* class is not present.