This is a program I ended up doing for a homework assignment back in the day. It does serve some practical purpose on occasion, but is also a great learning tool for recursion. I did take a little bit of time to go back through and have the program handle positive and negative doubles and quite large ones at that. One practical application you might see this used in could be check writing. 

This is a C# Console Application code so simply load up visual studios (or your favorite C# compiler) and drop this code on into a Console application .cs file. Click Run, and you're ready to start converting.

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

namespace NumWords
{
    class Program
    {
        // PROGRAM HANDLES NEGATIVE AND POSITIVE DOUBLES


        static String NumWordsWrapper(double n)
        {
            string words = "";
            double intPart;
            double decPart = 0;
            if (n == 0)
                return "zero";
            try {
                string[] splitter = n.ToString().Split('.');
                intPart = double.Parse(splitter[0]);
                decPart = double.Parse(splitter[1]);
            } catch {
                intPart = n;
            }

            words = NumWords(intPart);

            if (decPart > 0) {
                if (words != "")
                    words += " and ";
                int counter = decPart.ToString().Length;
                switch (counter) {
                    case 1: words += NumWords(decPart) + " tenths"; break;
                    case 2: words += NumWords(decPart) + " hundredths"; break;
                    case 3: words += NumWords(decPart) + " thousandths"; break;
                    case 4: words += NumWords(decPart) + " ten-thousandths"; break;
                    case 5: words += NumWords(decPart) + " hundred-thousandths"; break;
                    case 6: words += NumWords(decPart) + " millionths"; break;
                    case 7: words += NumWords(decPart) + " ten-millionths"; break;
                }
            }
            return words;
        }

        static String NumWords(double n) //converts double to words
        {
            string[] numbersArr = new string[] { "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen" };
            string[] tensArr = new string[] { "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninty" };
            string[] suffixesArr = new string[] { "thousand", "million", "billion", "trillion", "quadrillion", "quintillion", "sextillion", "septillion", "octillion", "nonillion", "decillion", "undecillion", "duodecillion", "tredecillion", "Quattuordecillion", "Quindecillion", "Sexdecillion", "Septdecillion", "Octodecillion", "Novemdecillion", "Vigintillion" };
            string words = "";

            bool tens = false;

            if (n < 0) {
                words += "negative ";
                n *= -1;
            }

            int power = (suffixesArr.Length + 1) * 3;

            while (power > 3) {
                double pow = Math.Pow(10, power);
                if (n >= pow) {
                    if (n % pow > 0) {
                        words += NumWords(Math.Floor(n / pow)) + " " + suffixesArr[(power / 3) - 1] + ", ";
                    } else if (n % pow == 0) {
                        words += NumWords(Math.Floor(n / pow)) + " " + suffixesArr[(power / 3) - 1];
                    }
                    n %= pow;
                }
                power -= 3;
            }
            if (n >= 1000) {
                if (n % 1000 > 0) words += NumWords(Math.Floor(n / 1000)) + " thousand, ";
                else words += NumWords(Math.Floor(n / 1000)) + " thousand";
                n %= 1000;
            }
            if (0 <= n && n <= 999) {
                if ((int)n / 100 > 0) {
                    words += NumWords(Math.Floor(n / 100)) + " hundred";
                    n %= 100;
                }
                if ((int)n / 10 > 1) {
                    if (words != "")
                        words += " ";
                    words += tensArr[(int)n / 10 - 2];
                    tens = true;
                    n %= 10;
                }

                if (n < 20 && n > 0) {
                    if (words != "" && tens == false)
                        words += " ";
                    words += (tens ? "-" + numbersArr[(int)n - 1] : numbersArr[(int)n - 1]);
                    n -= Math.Floor(n);
                }
            }

            return words;

        }
        static void Main(string[] args)
        {
            Console.Write("Enter a number to convert to words: ");
            Double n = Double.Parse(Console.ReadLine());

            Console.WriteLine("{0}", NumWordsWrapper(n));
        }
    }
}

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