title: Why "plus" sign for ID resources in referred names in Android
body: <p>I couldn't come up with a better title for this question, so please feel free to amend it if you can.</p>
  
  <p>So here is my question. I decided to write a small app for myself and along the way I am also trying to learn Android Development better. I know why there is a <a href="http://developer.android.com/guide/topics/ui/declaring-layout.html#id" rel="nofollow"><code>+</code>(plus)</a> sign in the <code>android:id</code> attribute as it means "creating a new id resource". But I don't know why Android needs to put <code>+</code> in the ones used for referencing such as below:</p>
  
  <p>Here I have two buttons named <code>StartButton</code> and <code>StopButton</code> and as they are new resources, the <code>android:id</code> contains <code>+</code>:</p>
  
  <pre><code>  &lt;Button
      android:id="@+id/StartButton"
      android:layout_width="150dp"
      android:layout_height="wrap_content"
      android:layout_alignParentLeft="true"
      android:layout_below="@+id/textView1"
      android:layout_marginTop="53dp"
      android:textSize="18dp"
      android:text="Start"
      android:onClick="StartButton_OnClick" /&gt;
  
  &lt;Button
      android:id="@+id/StopButton"
      android:layout_width="150dp"
      android:layout_height="wrap_content"
      android:layout_alignTop="@+id/StartButton"
      android:layout_toRightOf="@+id/StartButton"
      android:text="Stop"
      android:textSize="18dp"
      tools:context=".CallerBlockerActivity"
      android:onClick="StopButton_OnClick" /&gt;
  </code></pre>
  
  <p>But why <code>android:layout_alignTop</code> contains <code>+</code> puzzles me since <code>id/StartButton</code> is defined when <code>@id/StopButton</code> reaches it. Can some one please explain the necessity of this syntax?</p>