ActionScript 3.0 :: Code Doesn't Define Position Variable?

I run a demo code,it extends Sprite,like follows:

public class Vehicle extends Sprite{   protected var _position:Vector2D;   public function Vehicle(){     _position = new Vector2D();    draw();   }   protected function draw():void{     graphics.clear();     graphics.lineStyle(0); [code]....
 
I don't understand following code,because I only define _position and don't define position variable,why I can use position.x?     x = position.x;     y = position.y;