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 secrets. Ciphers(暗码) are another. In the code each word is written as a secret code or word number. In a cipher each letter is changed.

Codes and ciphers have played an important role in the history of the word. Julius Caesar, the Roman ruler who defeated almost all the countries in Europe about 2000years ago, used a cipher when he sent secret messages to his troops. During American Revolution, George Washington’s spies used a kind of code to send him information about the enemy before his military（军事的） action. In World War 2,the American “broke”, or figured out, Japan’s most important navy codes and got enough information to destroy a powerful Japanese fleet（舰队）。

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. Businessman 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（星相家）,mathematician, and doctor, invented the trellis ciphers. 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 the clean sheet of paper and wrote the secret message through the holes. Then he removed the trellis and filed the rest of the paper with words that would make sense. When his friends received it, He put his trellis over the writing and read the secret message.

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