Is 'window' really global in JavaScript?

Take this piece of JavaScript code in a browser:

<script>
    console.log(window.someThing);
    var x = 12;

    function foo() {
       window.otherThing = x;
    }
</script>

Inside foo we can access window. We all know that, but why exactly?

• Is it some kind of special global variable?
• Or does the "root scope" (inside the script tag) have it as an implicit local variable and is it simply "closure-inherited" as any other local variable (like x above) can be?

And how does that concur with variables declared directly inside the script tag being set as properties of window? (Or is that not so?)

<script>
    var x = 12;
    function() {
       console.log(window.x);
    }
</script>