LinearLayout android:layout_gravity= "Bottom" in Android layout does not work

In the interface layout, we often use is linearlayout, but sometimes it is not convenient to use, for example, we want to put a button on the page at the bottom of the screen, when we set android:layout_gravity= "Bottom" is not effective. Why is it? for LinearLayout:

< Span style= "FONT-SIZE:14PX; Background-color:rgb (255,255,255) "> when android:orientation= "vertical"   , only the horizontal setting will work, and the vertical setting will not work. That is: The left,right,center_horizontal is in force.
      when android:orientation=" horizontal ", only the vertical setting will work, and the horizontal setting will not work. That is: The top,bottom,center_vertical is in force.   

< Span style= "FONT-SIZE:14PX; Background-color:rgb (255,255,255) ">       If we're going to implement a view on the phone screen at the bottom, we can use relativelayout in the outermost (or root) layout of the XML we're laying out. Again in Relativelayout contains the linearlayout we have written, The view to be displayed at the bottom of the phone screen is added with the attribute android:layout_alignparentbottom=" true ", so it's OK.

LinearLayout android:layout_gravity= "Bottom" in Android layout does not work