c# overlapping class question!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

using System;

class line
{
    Public class point // What is the reason why public should be included in this part?
    {
        public int x;
    }
    public point starting = new point();
}
class C_sharp_book
{
    public static void Main()
    {
        line mine = new line();

        mine.starting.x = 3;
        Console.WriteLine("{0}", mine.starting.x);
    }
}