Understanding the grid classes ( col-sm-# and col-lg-# ) in Bootstrap 3

I'm getting started on Bootstrap 3 and I'm having some trouble understanding how the grid classes are meant to be used.

Here's what I've figured out so far:

It appears that the classes col-sm-# and col-lg-# differ from plain old col-# in that they will only apply when screens are above a certain size (768px and 992px respectively). If you omit the -sm- or -lg- the divs will never collapse into one column.

However, when I create two divs inside a row that are both col-sm-6 it seems they are only side by side when the window is between 768px and 992px wide. In other words, if I shrink the window all the way down and then slowly widen it, the layout is single column, then two columns, then back to single column again.

1. Is this the intended behavior?
2. If I want two columns for anything over 768px, should I apply both classes? (<div class="col-sm-6 col-lg-6">)
3. Should col-6 also be included? <div class="col-6 col-sm-6 col-lg-6">

[UPDATE BELOW]

I took another look at the docs and it appears I overlooked a section which talks specifically about this.

The answers to my questions:

1. Yes, they are meant to apply only to specific ranges, rather than everything above a certain width.

2. Yes, the classes are meant to be combined.

3. It appears that this is appropriate in certain cases but not others because the col-# classes are basically equivalent to col-xsm-# or, widths above 0px (all widths).

Other than reading the docs too quickly, I think I was confused because I came into Bootstrap 3 with a "Bootstrap 2 mentality". Specifically, I was using the (optional) responsive styles (bootstrap-responsive.css) in v2 and v3 is quite different (for the better IMO).

UPDATE for stable release:

This question was originally written when RC1 was out. They made some major changes in RC2 so for anyone reading this now, not everything mentioned above still applies.

As of when I'm currently writing this, the col-*-# classes DO seem to apply upwards. So for example, if you want an element to be 12 columns (full width) for phones, but two 6 columns (half page) for tablets and up, you would do something like this:

<div class="col-xs-12 col-sm-6"> ... //NO NEED FOR col-md-6 or col-lg-6

(They also added an additional xs break point after this question was written.)

Here you have a very good tutorial, that explains, how to use the new grid classes in Bootstrap 3.

It also covers mixins etc.

> "If I want two columns for anything over 768px, should I apply both classes?"

This should be as simple as:

<div class="row">
      <div class="col-sm-6"></div>
      <div class="col-sm-6"></div>    
</div>

No need to add the col-lg-6 too.

Demo: http://www.bootply.com/73119

The best way to understand is to simply think from top to bottom ( Large Desktops to Mobile Phones)

Firstly, as B3 is mobile first so if you use xs then the columns will be same from Large desktops to xs ( i recommend using xs or sm as this will keep everything the way you want on every screen size )

Secondly if you want to give different width to columns on different devices or resolutions, than you can add multiple classes e.g

the above will change the width according to the screen resolutions, REMEMBER i am keeping the total columns in each class = 12

I hope my answer would help!

To amend SDP's answer above, you do NOT need to declarecol-xs-12 in <div class="col-xs-12 col-sm-6">. Bootstrap 3 is mobile-first, so every div column is assumed to be a 100% width div by default - which means at the "xs" size it is 100% width, it will always default to that behavior regardless of what you set at sm, md, lg. If you want your xs columns to be not 100%, then you normally do a col-xs-(1-11).

This might be late as I think most of us are using BS4. This article explained all the questions you asked in a detailed and simple manner also includes what to do when. The detailed guide to use bs4 or bootstrap

https://uxplanet.org/how-the-bootstrap-4-grid-works-a1b04703a3b7