Inconsistent accessibility: base class is less accessible than child class

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C#

I got a book about C# programming then copied and pasted some code as an example.
How ever the error "Inconsistent accessibility: base class is less accessible than child class" pop out.

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

namespace _2_52
{
    class Program
    {
        static void Main(string[] args)
        {
            Rectangle rectangle = new Square(12);
            rectangle.Width = 10;
            rectangle.Height = 5;
            Console.WriteLine(rectangle.Area);
        }
    }
    class Rectangle
    {
        public Rectangle(int width,int height)
        {
            Width = width;
            Height = height;
        }
        public virtual int Height { get; set; }
        public virtual int Width { get; set; }
        public int Area
        {
            get { return Height * Width; }
        }
    }
    private class Square : Rectangle
    {
        public Square(int size):base(size,size)
        {

        }
        public override int Width
        {
            get
            {
                    return base.Width;
            }
            set
            {
                base.Width = value; base.Width = value;
            }
        }
    }
}

Thanks for correcting it.