Why use double alias instead Double

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace UseDoubleAlias
{
    class Program
    {
        static void Main(string[] args)
        {
            object value = 23.5;

            // "is" operator checks if given value can be converted to
            // given type.
            // Converting from object to other type - unboxing.
            // Note: Unboxing is one of the supported operations of
            // "is" operator.
            // See here for more on "is": http://msdn.microsoft.com/en-us/library/scekt9xw%28v=vs.71%29.aspx
            if (value is Double)
            {
                Console.WriteLine(value + " is Double.");
            }
            else
            {
                Console.WriteLine(value + " is not Double.");
            }

            if (value is System.Double)
            {
                Console.WriteLine(value + " is System.Double.");
            }
            else
            {
                Console.WriteLine(value + " is not System.Double.");
            }
        }
    }

    class Double
    {
        // Empty class(type) having the same name as System.Double.
    }
}