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In this chapter we shall discuss the Bootstrap Grid System.

What is a Grid?

As put by wikepedia −
In graphic design, a grid is a structure (usually two-dimensional) made up of a series of intersecting straight (vertical, horizontal) lines used to structure the content. It is widely used to design layout and content structure in print design. In web design, it is a very effective method to create a consistent layout rapidly and effectively using HTML and CSS.
To put in simple words, grids in web design organise and structure content, makes the websites easy to scan and reduces the cognitive load on users.

What is Bootstrap Grid System?

As put by the official documentation of Bootstrap for grid system −
Bootstrap includes a responsive, mobile first fluid grid system that appropriately scales up to 12 columns as the device or viewport size increases. It includes predefined classes for easy layout options, as well as powerful mixins for generating more semantic layouts.
Let us understand the above statement. Bootstrap 3 is mobile first in the sense that the code for Bootstrap now starts by targeting smaller screens like mobile devices, tablets, and then “expands” components and grids for larger screens such as laptops, desktops.

Mobile First Strategy

  • Content
    • Determine what is most important.
  • Layout
    • Design to smaller widths first.
    • Base CSS address mobile device first; media queries address for tablet, desktops.
  • Progressive Enhancement
    • Add elements as screen size increases.

Working of Bootstrap Grid System

Grid systems are used for creating page layouts through a series of rows and columns that house your content. Here's how the Bootstrap grid system works −
  • Rows must be placed within a .container class for proper alignment and padding.
  • Use rows to create horizontal groups of columns.
  • Content should be placed within the columns, and only columns may be the immediate children of rows.
  • Predefined grid classes like .row and .col-xs-4 are available for quickly making grid layouts. LESS mixins can also be used for more semantic layouts.
  • Columns create gutters (gaps between column content) via padding. That padding is offset in rows for the first and the last column via negative margin on .rows.
  • Grid columns are created by specifying the number of twelve available columns you wish to span. For example, three equal columns would use three .col-xs-4.

Media Queries

Media query is a really fancy term for "conditional CSS rule". It simply applies some CSS, based on certain conditions set forth. If those conditions are met, the style is applied.
Media Queries in Bootstrap allow you to move, show and hide content based on the viewport size. Following media queries are used in LESS files to create the key breakpoints in the Bootstrap grid system.
/* Extra small devices (phones, less than 768px) */
/* No media query since this is the default in Bootstrap */

/* Small devices (tablets, 768px and up) */
@media (min-width: @screen-sm-min) { ... }

/* Medium devices (desktops, 992px and up) */
@media (min-width: @screen-md-min) { ... }

/* Large devices (large desktops, 1200px and up) */
@media (min-width: @screen-lg-min) { ... }
Occasionally these are expanded to include a max-width to limit CSS to a narrower set of devices.
@media (max-width: @screen-xs-max) { ... }
@media (min-width: @screen-sm-min) and (max-width: @screen-sm-max) { ... }
@media (min-width: @screen-md-min) and (max-width: @screen-md-max) { ... }
@media (min-width: @screen-lg-min) { ... }
Media queries have two parts, a device specification and then a size rule. In the above case, the following rule is set −
Let us consider this line −
@media (min-width: @screen-sm-min) and (max-width: @screen-sm-max) { ... }
For all devices no matter what kind with min-width: @screen-sm-min if the width of the screen gets smaller than @screen-sm-max, then do something.

Grid options

The following table summarizes aspects of how Bootstrap grid system works across multiple devices −

 Extra small devices Phones (<768px) Small devices Tablets (≥768px) Medium devices Desktops (≥992px) Large devices Desktops (≥1200px)
Grid behavior Horizontal at all times Collapsed to start, horizontal above breakpoints Collapsed to start, horizontal above breakpoints Collapsed to start, horizontal above breakpoints
Max container width None (auto) 750px 970px 1170px
Class prefix .col-xs- .col-sm- .col-md- .col-lg-
# of columns 12 12 12 12
Max column width Auto 60px 78px 95px
Gutter width 30px
(15px on each side of a column)
 30px
(15px on each side of a column)
 30px
(15px on each side of a column)
 30px
(15px on each side of a column)
Nestable Yes Yes Yes Yes
Offsets Yes Yes Yes Yes
Column ordering Yes Yes Yes Yes

Basic Grid Structure

Following is basic structure of Bootstrap grid −
<div class = "container">
   
   <div class = "row">
      <div class = "col-*-*"></div>
      <div class = "col-*-*"></div>
   </div>
   
   <div class = "row">...</div>
 
</div>

<div class = "container">
   ....
</div>
Let us see some simple grid examples −

Responsive column resets

With the four tiers of grids available, you are bound to run into issues where at certain breakpoints, the columns don't clear quite right as one is taller than the other. To fix that, use a combination of a class .clearfix and the responsive utility classes as shown in the following example −
<div class = "container">
   <div class = "row" >
   
      <div class = "col-xs-6 col-sm-3" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit.</p>
      </div>
      
      <div class = "col-xs-6 col-sm-3" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do 
            eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut 
            enim ad minim veniam, quis nostrud exercitation ullamco laboris 
            nisi ut aliquip ex ea commodo consequat.</p>
         
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do 
            eiusmod tempor incididunt ut.</p>
      </div>

      <div class = "clearfix visible-xs"></div>
      
      <div class = "col-xs-6 col-sm-3" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <p>Ut enim ad minim veniam, quis nostrud exercitation ullamco 
            laboris nisi ut aliquip ex ea commodo consequat.</p>
      </div>
      
      <div class = "col-xs-6 col-sm-3" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do 
            eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut 
            enim ad minim</p>
      </div>
      
   </div>
</div>
This will produce the following result −
Resize your viewport or check it out on your phone for a desired result of this example.

Offset Columns

Offsets are a useful feature for more specialized layouts. They can be used to push columns over for more spacing, (for example). The .col-xs = * classes don’t support offsets, but they are easily replicated by using an empty cell.
To use offsets on large displays, use the .col-md-offset-* classes. These classes increase the left margin of a column by * columns where * range from 1 to 11.
In the following example, we have <div class = "col-md-6">..</div>, We will center this using class .col-md-offset-3.
<div class = "container">

   <h1>Hello, world!</h1>

   <div class = "row" >
      <div class = "col-xs-6 col-md-offset-3" style = "background-color: #dedef8; 
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit.</p>
      </div>

   </div>
 
</div>
This will produce the following result −

Nesting columns

To nest your content with the default grid, add a new .row and set of .col-md-* columns within an existing .col-md-* column. Nested rows should include a set of columns that add up to 12.
In the following example, the layout has two columns, with the second one being split into four boxes over two rows.
<div class = "container">
   <h1>Hello, world!</h1>

   <div class = "row">

      <div class = "col-md-3" style = "background-color: #dedef8; 
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <h4>First Column</h4>
         <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit.</p>
      </div>

      <div class = "col-md-9" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         <h4>Second Column- Split into 4 boxes</h4>
         <div class = "row">
            
            <div class = "col-md-6" style = "background-color: #B18904;
               box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
               
               <p>Consectetur art party Tonx culpa semiotics. 
                  Pinterest assumenda minim organic quis.</p>
            </div>
            
            <div class = "col-md-6" style = "background-color: #B18904;
               box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
               
               <p>sed do eiusmod tempor incididunt ut labore et dolore magna 
                  aliqua. Ut enim ad minim veniam, quis nostrud exercitation 
                  ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
            </div>
    
         </div>

         <div class = "row">
   
            <div class = "col-md-6" style = "background-color: #B18904;
               box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
               
               <p>quis nostrud exercitation ullamco laboris nisi ut aliquip 
                  ex ea commodo consequat.</p>
            </div>   
            
            <div class = "col-md-6" style = "background-color: #B18904;
               box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
               
               <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, 
                  sed do eiusmod tempor incididunt ut labore et dolore magna 
                  aliqua. Ut enim ad minim.</p>
            </div>
    
         </div>

      </div>

   </div>
 
</div>
This will produce the following result −

Column Ordering

Another nice feature of Bootstrap grid system is that you can easily write the columns in an order, and show them in another one. You can easily change the order of built-in grid columns with .col-md-push-* and .col-md-pull-* modifier classes where * range from 1 to 11.
In the following example we have two columns layout with left column being the narrowest and acting as a sidebar. We will swap the order of these columns using .col-md-push-* and .col-md-pull-* classes.
<div class = "container">
   <h1>Hello, world!</h1>
   
   <div class = "row">
      <p>Before Ordering</p>
      
      <div class = "col-md-4" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         I am on left
      </div>
      
      <div class = "col-md-8" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         I am on right
      </div>
      
   </div>
 
   <br>
   
   <div class = "row">
      <p>After Ordering</p>
      
      <div class = "col-md-4 col-md-push-8" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         I was on left
      </div>
      
      <div class = "col-md-8 col-md-pull-4" style = "background-color: #dedef8;
         box-shadow: inset 1px -1px 1px #444, inset -1px 1px 1px #444;">
         
         I was on right
      </div>
  
   </div>

</div>
This will produce the following result −

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