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public static class Numeric
{
  private enum Digit 
  { 
    zero = 0, one = 1, two = 2, three = 3, four = 4, 
    five = 5, six = 6, seven = 7, eight = 8, nine = 9 
  }

  private enum Teen 
  { 
    ten = 10, eleven = 11, twelve = 12, thirteen = 13, fourteen = 14, 
    fifteen = 15, sixteen = 16, seventeen = 17, eighteen = 18, nineteen = 19 
  }

  private enum Ten 
  { 
    twenty = 2, thirty = 3, forty = 4, fifty = 5, 
    sixty = 6, seventy = 7, eighty = 8, ninety = 9 
  }

  private enum PowerOfTen 
  { 
    hundred = 0, thousand = 1, million = 2, billion = 3, 
    trillion = 4, quadrillion = 5, quintillion = 6 
  }

  /// <summary>
  /// How many powers of ten there are; faster to work this out ahead of time,
  /// and I didn't want to hard-code it into the algorithm...
  /// </summary>
  private static int PowersOfTen = Enum.GetValues(typeof(PowerOfTen)).Length;

  /// <summary>
  /// Converts a number to English words
  /// </summary>
  /// <param name="N">The number</param>
  /// <returns>The number, in English</returns>
  public static string ToWords(double N)
  {
    string Prefix = N < 0 ? "minus " : "";
    string Significand = Digit.zero.ToString();
    string Mantissa = "";
    if ((N = Math.Abs(N)) > 0)
    {
      // Do the Mantissa
      if (N != Math.Floor(N))
      {
        Mantissa = " point";
        foreach (char C in N.ToString().Substring(N.ToString().IndexOf('.') + 1))
          Mantissa += " " + ((Digit)(int.Parse(C.ToString())));
      }

      // Figure out the bit of the Significand less than 100
      int n = Convert.ToInt32(N = Math.Floor(N)) % 100;
      Significand = n == 0 ? ""
        : n < 10 ? ((Digit)n).ToString()
        : n < 20 ? ((Teen)n).ToString()
        : (Ten)(n / 10) + "-" + (Digit)(n % 10);

      // Do the other powers of 10, if there are any
      if ((N = Math.Floor(N / 100D)) > 0)
      {
        string EW = "";
        for (int i = 0; (N > 0) && (i < PowersOfTen); i++)
        {
          double p = Math.Pow(10, (i << 1) + 1);
          n = Convert.ToInt32(N % p);
          if (n > 0)
            EW = ToWords(n) + " " + (PowerOfTen)i + (EW.Length == 0 ? "" : ", " + EW);
          N = Math.Floor(N / p);
        }
        if (EW.Length > 0)
          Significand = EW + (Significand.Length == 0 ? "" : " and " + Significand);
      }
    }
    return Prefix + (Significand + Mantissa).Trim();
  }
}