_Signal practice._--Full efficiency of the signaler can be maintained only through constant practice, and those in charge of Signal Corps troops should see that sufficient practice be had to insure that accuracy and rapidity in handling messages which is so essential in time of war. Instruction should commence with the study of the principles of signaling and the theories of their general use, and the pupil should be well grounded in this study before practice is begun. He should so memorize the alphabets to be used that no letter combination will require thought to determine its meaning. Daily inspections should be made to insure that all signaling instruments, appliances, and materials are in readiness for instant use. Defects in the apparatus annoy the sender; to a greater extent they annoy the person to whom the messages are imperfectly sent, and delays result that may have serious consequences. CHAPTER VI. CODES AND CIPHERS. A code is a list or collection of arbitrary words or groups of letters to each of which some ordinary word, proper name, phrase, or sentence is assigned for meaning. Ciphers embrace all means whereby writings may be transcribed into occult terms. All ciphers employ some distinct method for transcription, which method is termed a key. In practice the key is usually applied directly in enciphering and reversed in deciphering messages. CODES IN USE. The codes of the Western Union and Postal Telegraph companies are examples of well-known codes suited to general commercial use. Besides these, many special codes have been formulated, so as to embody technical expressions especially adapted to use in particular lines of industry. The War Department Code is a military code adapted to the special needs of the military establishment in peace and war. EMPLOYMENT OF CODES. Codes are primarily intended for economy, but they may also be readily employed to secure secrecy. When used solely for economy, the coded message is said to be plain code; that is, the word or phrases of the message are coded by direct reference to their respective code equivalents. Thus plain code is readily translatable to anyone in possession of a code book. When secrecy is desired, some method of enciphering or key is employed in such a way that only persons in possession of it can in conjunction with the code book decipher it. In such case the message is said to be in cipher code. CIPHER CODE.