In all codes each expression and its equivalent in plain language is assigned a number. These numbers usually commence at unity and increase consecutively to any desired figure. Messages may be enciphered by means of a key number or series of numbers. An additive number, say 55 additive, requires that in enciphering a message, the fifty-fifth word numerically greater than the proper code word shall be used; if 55 subtractive is used, the fifty-fifth word numerically smaller than the proper code word is to be used. By agreement a single key number can be used alternately additive and subtractive, that is, first additive, second subtractive, third additive, etc. The key numbers are used over and over until the entire message is enciphered. The key number can sometimes be expressed by a single word, as, for instance, "Grant," each letter having a value of tens in accordance with its position in the alphabet; that is, G, the seventh letter equals 70; R equals 180; A equals 10; N equals 140; and T equals 200. Or by preconcerted arrangement letters may represent units or hundreds. Security from translation by persons not having the key number is greater when the key numbers are used alternately additive and subtractive. If a cipher key word is used, it should be one of an odd number of letters, as, for instance, "Jones," the numbers corresponding to the positions of the letters in the alphabet. The first number should be additive, the second subtractive, etc. By this means the first letter of the key word is additive the first time it is used, subtractive the second, additive the third, and so on. In some instances the key number, when added to or subtracted from the code number, gives a resulting number exceeding the highest code number or less than unity. In cases of this kind it should be remembered in enciphering that unity follows the highest code number in addition, and that the highest code number follows unity in subtraction. In deciphering a message the process of enciphering is reversed. THE WAR DEPARTMENT CODE. As previously stated, the War Department Code is the technical military code and contains expressions numbered consecutively from 1 to 62,000. All the code words are composed of 6 letters, which are so arranged that the vowels and consonants invariably alternate. In the formation of code words the following 13 letters only are used, viz, A, B, D, E, F, G, I, K, M, N, S, U, and X. The body of the code book is arranged as follows: (_a_) Army list, containing the name of every commissioned officer in the regular establishment. (_b_) Military organizations, giving all batteries, companies, troops, etc. (_c_) Military posts and stations, covering Alaska, Hawaii, Philippine Islands, Porto Rico, and the United States. (_d_) United States naval stations and vessels. (_e_) Geographical names. (_f_) Miscellaneous tables as follows: