I get a little giddy when I come across perfect uses for :nth-child or :nth-of-type (read about the difference). The better you understand them, the more css nerdgasms you get to have! In these simple "recipes" (really: expressions) I'll arbitrarily use a flat list of list items and randomly chosen numbers. But it should be fairly obvious how to…

style.less

body{
 font-family: 'Exo 2', sans-serif;
  text-align:center;
}
.s3-containers{
  display:inline-block;
  width:100%;
  margin-bottom:1em;
  text-align:center;
  border-bottom:solid 1px #ccc;
  h2{
    color:white;
    background:green;
    display:inline-block;
    padding:0.5em;
  }
  ul{
    li{
      list-style-type:none;
      display:inline-block;
      padding:0.5em;
      margin:0.3em;
    }// li
  } // ul
}
.s3-container1{
  ul{
    li{
      &:nth-child(5){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container1

.s3-container2{
  ul{
    li{
      &:nth-child(n+6){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container2
.s3-container3{
  ul{
    li{
      &:nth-child(-n+5){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container3
.s3-container4{
  ul{
    li{
      &:nth-child(4n+1){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container4
.s3-container5{
  ul{
    li{
      &:nth-child(odd){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container5
.s3-container6{
  ul{
    li{
      &:nth-child(even){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container6
.s3-container7{
  ul{
    li{
      &:first-child{
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container7
.s3-container8{
  ul{
    li{
      &:last-child{
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container8
.s3-container9{
  ul{
    li{
      &:nth-last-child(2){
        background: green;
        color:white;
      }
    }// li
  } // ul
} // s3-container9

Useful :nth-child Recipes

<link href='http://fonts.googleapis.com/css?family=Exo+2' rel='stylesheet' type='text/css'>
<h2>Useful :nth-child Recipes <small>with lESS</small></h2>
<div class="s3-containers s3-container1">
  <h2>Select Only the Fifth Element</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container2">
  <h2>Select All but the first five</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container3">
  <h2>Select Only First Five</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container4">
  <h2>Select Every Fourth, Starting At The First</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container5">
  <h2>Select Only Odd</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container6">
  <h2>Select Only Even</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>

<div class="s3-containers s3-container7">
  <h2>Select Only First Child</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container8">
  <h2>Select Only Last Child</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>
<div class="s3-containers s3-container9">
  <h2>Select the Second to Last Element</h2>
  <ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
    <li>4</li>
    <li>5</li>
    <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
    <li>10</li>
  </ul>
</div>