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 (暗码) are another. In a code, each word is written as a secret code or 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 him information about the enemy before his military action. In the Second World War, the Americans “broke” 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 was paid for the goods or when they were 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 mathematician and doctor, invented an interesting kind of trellis cipher.