Dynamically I create a set of boxes with slightly different height and want to appear 2, 3 or 4 of them (depending on the screen size) at the same row. I tried the following markup with bootstrap:

<div class="container-fluid">
  <div class="row-fluid">
    <div class="col-xs-6">
      Two<br>Lines
    </div>
    <div class="col-xs-6">
      Two<br>Lines
    </div>
    <div class="col-xs-6" style="background-color: red">
      Three<br>Lines<br>Jup
    </div>
    <div class="col-xs-6">
      Two<br>Lines
    </div>
    <div class="col-xs-6">
      Two<br>Lines
    </div>
    <div class="col-xs-6">
      Two<br>Lines
    </div>
  </div>
</div>

My problem is the space below my third column.

 A  B
 C  D
 C  E <- Should wrap to next row, because of C
 F  G

Can you tell how to achieve this? I recognized the advice to use a clearfix but I guess this attempt will cause problems and ugly code when using a different count of columns.

Thanks for your answers.

The problem

The problem is that all bootstrap columns try to float left.

From MDN on floats:

when an element is floated it is taken out of the normal flow of the document. It is shifted to the left or right until it touches the edge of its containing box or another floated element.

So when you have uneven heighted elements, the correct behavior is to stack them to the side.

Luckily, there are a lot of way to correct this to the anticipated behavior:

---

Using CSS Clear property

From MDN on Clear:

The clear CSS property specifies whether an element can be next to floating elements that precede it or must be moved down (cleared) below them

For this exact structure, you can apply clear to every other row using the nth-child selector:

<!-- language: lang-css -->

.col-xs-6:nth-child(odd) {
    clear: both;
}

Note: The nth-child selector won't work in IE8

Demo in jsFiddle / Stack Snippets:

<!-- begin snippet: js hide: true --> <!-- language: lang-css -->

.col-xs-6:nth-child(odd) {
  clear: left;
}
.col-xs-6 {
  border: 1px solid grey;
}

<!-- language: lang-html -->

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="container">
  <div class="row">
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6 label-warning">
      Three<br/>
      Lines<br/>
      Jump
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>
</div>

<!-- end snippet -->

---

Using .clearfix

The Bootstrap way to do this is to use the clearfix class which applies the following styles:

<!-- language: lang-css -->

.clearfix:before,
.clearfix:after {
  content: " ";
  display: table;
}
.clearfix:after {
  clear: both;
}

Note: You can use selectively target different screen sizes by combining this with the responsive utility classes (i.e. .visible-*-*, .hidden-*)

Demo in jsFiddle / Stack Snippets:

<!-- begin snippet: js hide: true --> <!-- language: lang-css -->

.col-xs-6 {
  border: 1px solid grey;
}

<!-- language: lang-html -->

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="container">
  <div class="row">
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    
    <div class="clearfix"></div>
    
    <div class="col-xs-6 label-warning">
      Three<br/>
      Lines<br/>
      Jump
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    
    <div class="clearfix"></div>
    
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>
</div>

<!-- end snippet -->

---

Using .row

You can also just wrap every pair of columns into a new row. This is the least reusable, but if every set of items forms a logical group, it does create semantic markup.

Demo in jsFiddle / Stack Snippets:

<!-- begin snippet: js hide: true --> <!-- language: lang-css -->

.col-xs-6 {
  border: 1px solid grey;
}

<!-- language: lang-html -->

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="container">
  
  <div class="row">
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>

  <div class="row">
    <div class="col-xs-6 label-warning">
      Three<br/>
      Lines<br/>
      Jump
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>
  
  <div class="row">
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>
  
</div>

<!-- end snippet -->

---

Fixing the height

Depending on your use case, you might just benefit from making everything the same height. This would work well if you had similar content in every column and wanted a consistent looking grid.

<!-- language: lang-css -->

.col-xs-6 {
  height: 7rem;
}

Demo in jsFiddle / Stack Snippets:

<!-- begin snippet: js hide: true --> <!-- language: lang-css -->

.col-xs-6 {
  height: 7rem;
}
.col-xs-6 {
  border: 1px solid grey;
}

<!-- language: lang-html -->

<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div class="container">
  <div class="row">
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6 label-warning">
      Three<br/>
      Lines<br/>
      Jump
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
    <div class="col-xs-6">
      Two<br/>
      Lines
    </div>
  </div>
</div>

<!-- end snippet -->