If you and your friends wish to share a secret, you can write it in code, and no one else will be able to read it. Codes are one way of writing in secret. Ciphers   are another. In a code each word is written as a secret code word or a code number. In a cipher each letter is changed. Codes and ciphers have played an important role in the history of the world. Julius Caesar, the Roman ruler who defeated almost all the countries in Europe about 2,000 years ago, used a cipher when he sent secret messages to his troops. During the American Revolution, George Washington's spies used a kind of code to send his information about the enemy before his military action. In World War II, the Americans"broke"or figured out Japan's most important navy codes and got enough information to destroy a powerful Japanese fleet. Storekeepers use codes to mark their goods. The codes show how much is paid for the goods or when they are added to the stock. Businessmen use codes to hide plans from their business enemies. Sometimes personal letters or diaries are written in code. Many people enjoy figuring out codes and ciphers simply as a hobby. In the 16th century, codes and ciphers were very popular among scientists. They wrote messages to each other in code so that no one else would learn their secrets. Geronimo Gardano, an Italian astrologer  , mathematician, and doctor, invented the trellis cipher. He took two sheets of paper and cut exactly the same holes in each one. Then he sent one sheet, which he called a trellis, to a friend and kept the other for himself. Whenever he wanted to write a message, he put his trellis over a clean sheet of paper and wrote the secret message through the holes. There he removed the trellis and filled the rest of the paper with words that would make sense. When his friend received it, he put his trellis over the writing and read the secret message. It is NOT mentioned in the passage that codes and ciphers are used for the purpose of   _  .
military affairs commercial secrets scientific achievements personal enjoyment