Complex nth child selection

Hey guys , i just saw some scary code , it uses the nth child selector , please have a look and explain it to me .

HTML ::

[CODE]
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
<div>6</div>
<div>7</div>
<div>8</div>
<div>9</div>
<div>10</div>
<div>11</div>
<div>12</div>
<div>13</div>
<div>14</div>
<div>15</div>
<div>16</div>
<div>17</div>
<div>18</div>
<div>19</div>
<div>20</div>
<div>21</div>
<div>22</div>
<div>23</div>
<div>24</div>
<div>25</div>
<div>26</div>
<div>27</div>
<div>28</div>
<div>29</div>
<div>30</div>
<div>31</div>
<div>32</div>
<div>33</div>
<div>34</div>
<div>35</div>
<div>36</div>
<div>37</div>
<div>38</div>
<div>39</div>
<div>40</div>
<div>41</div>
<div>42</div>
<div>43</div>
<div>44</div>
<div>45</div>
<div>46</div>
<div>47</div>
<div>48</div>
[/CODE]

CSS::

[CODE]
div:nth-child(12n – 9),
div:nth-child(12n – 7),
div:nth-child(12n – 5),
div:nth-child(12n – 2),
div:nth-child(12n) {
color: red;
}
[/CODE]

Fiddle:: [URL=”https://jsfiddle.net/mvve6kub/”]https://jsfiddle.net/mvve6kub/[/URL]

What does the

[CODE]12n – 9[/CODE]

, really mean ?

Gautam.

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