LETTERS OF INSTRUCTION, issued by higher commanders, regulate in a general way movements covering much time and space. Army and Corps commanders may, by visits and conferences, often communicate verbally the substance of letters of instruction. FIELD ORDERS, issued verbally or in writing by commanders of tactical units, regulate the tactical actions of troops, and such strategical actions as are not covered by letters of instruction. GENERAL ORDERS are written directions having to do with the general administration of a command. They contain the necessary details for carrying out injunctions from higher authority, for keeping up standing instructions, and for publishing results of special and general courts-martial. ORDERS (when not used as a general term) refer to the instructions communicated to troops by commanders of divisions and of separate brigades in regard to combat, field, ammunition, supply, sanitary, and engineer trains. SPECIAL ORDERS cover such matters in regard to individuals as are not necessary to be communicated to the entire command. OPERATION ORDERS, as we now use the term in reference to trench warfare, are the orders governing a certain enterprise along a given sector of the front. Although they are even more precise than field orders, they contain more details on account of the wide difference between trench and open warfare. CIRCULARS are printed from time to time by various bureaus and departments of the War Department, and by purchasing officers, for the publication of data pertaining to the business of a bureau or department. BULLETINS publish, usually in pamphlet form, information for the military service. Both _circulars_ and _bulletins_ have the force of orders. MEMORANDA are published from time to time by commanding officers to embody directions not vital or sweeping enough for general or special orders. _Memoranda_ have the force of orders. OFFICIAL CORRESPONDENCE consists of official letters, returns, rolls, estimates, requisitions, and certain routine reports. These types of communication have been defined in order to give a general survey of the many kinds of written and oral expression of which military men should become masters. Although the task appears huge, proficiency can be gained by attention to the hints suggested in Chapter I. In order to apply those suggestions in a practical way, we must now sort out from all of the above types those which should press themselves most upon our attention. Certainly all these types cannot be equally necessary for our immediate study. Possibly if we classify them from the point of view of the occasion for their use, we may see more clearly how to treat them. CLASSIFICATION ACCORDING TO STRESS MESSAGE } Communicated usually in the presence FIELD ORDERS } of the enemy under stress. OPERATION ORDERS } ORDERS—Written often in the presence of the enemy under stress.