How do I create a multiple-key dictionary?

namespace MultiKeyDictionary.Console {
    using System.Collections.Generic;
    using System;

    internal class Program {
        private static void Main(string[] args) {
            var dictionary = new Dictionary<Tuple<string,string>, string>();

            var key = new Tuple<string, string>("multiple", "keys");
            var value = "Open Sesame!";

            dictionary.Add(key, value);
            Console.WriteLine(dictionary[key]);
        }
    }

    public struct Tuple <T1, T2> {
        public readonly T1 Item1;
        public readonly T2 Item2;

        public Tuple(T1 item1, T2 item2) {
            Item1 = item1;
            Item2 = item2;
        }
    }
}