unable to use android:layout_alignBelow?

I'm on the verge of learning Android app development. I'm using Android studio. And I'm learning it with a book 'Learn Android App Development' from Apress. And my doubt is :

<TextView
     android:id="@+id/dataView2"
     android:layout_toRightOf="@+id/textView2"
     android:layout_alignStart="@+id/dataView1"
     android:layout_alignBelow="@+id/dataView1"    // <- This line
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:text="@string/planet_mass_label" />

While they've used android:layout_alignBelow in the text , when I use it in my IDE , It warns me that it is an unknown attribute. Where exactly does the fault lies? Is there any alternate way to invoke the same?