Why can’t you call CSS "pasted" classes

Question

Why can’t I call classes "glued", like this:

.div1.div2.div3 {
   blablabla;
}

That’s the only way I can do it:

.div1 .div2 .div3{
   blablabla
}

I’d like to know the difference.

Answer 1

At first:

.div1.div2.div3 {
   blablabla;
}

You are selecting an element that has all three classes, such as:

<div class="div1 div2 div3"></div>

In the second case you are working with the following structure:

<div class="div1">
  <div class="div2">
    <div class="div3">
      ...
    </div>
  </div>
</div>

That is div1 > div2 > div3, in relation to inheritance. Thus selecting the element .div3 son of the element .div2 son of the element .div1.