j0hnik,
//$(".item").index(this)
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.items-content > div{
display: none;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
var item = $(".item"), content = $(".item-content");
item.click(function() {
content.hide().eq(item.index(this)).fadeIn();
});
});
</script>
</head>
<body>
<div class="container">
<div class="item-box">
<div class="item">1</div>
<div class="item">2</div>
</div>
<div class="item-box">
<div class="item">3</div>
<div class="item">4</div>
</div>
<div class="item-box">
<div class="item">5</div>
<div class="item">6</div>
</div>
<div class="item-box">
<div class="item">7</div>
<div class="item">8</div>
</div>
....
</div>
<div class="items-content">
<div class="item-content">1</div>
<div class="item-content">2</div>
<div class="item-content">3</div>
<div class="item-content">4</div>
<div class="item-content">5</div>
<div class="item-content">6</div>
<div class="item-content">7</div>
<div class="item-content">8</div>
.....
</div>
</body>
</html>