title: nested LinearLayout
body: <p>I read this code in the Hello Android book and i dont know why to write two LinearLayout.
  I could be one of these remove or move to other?
  Why do we use nested LinearLayout? What does it do?</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout
      xmlns:android="http://schemas.android.com/apk/res/android"
      android:background="@color/background"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:padding="30dip"
      android:orientation="horizontal"&gt;
      &lt;LinearLayout
          android:orientation="vertical"
          android:layout_height="wrap_content"
          android:layout_width="fill_parent"
          android:layout_gravity="center"&gt;
          &lt;TextView
              android:text="@string/main_title"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:gravity="center"
              android:layout_marginBottom="25dip"
              android:textSize="24.5sp"/&gt;
          &lt;Button
              android:id="@+id/continue_button"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="@string/continue_label"/&gt;
          &lt;Button
              android:id="@+id/new_button"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="@string/new_game_label"/&gt;
          &lt;Button
              android:id="@+id/about_button"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="@string/about_label" /&gt;
          &lt;Button 
              android:id="@+id/exit_button"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:text="@string/exit_label"/&gt;
     &lt;/LinearLayout&gt;
  &lt;/LinearLayout&gt;
  </code></pre>
  
  <p>Please help me.
  Thank you.</p>