Advance Base Post Office (_Fig._ 34). [Illustration: 33 34] Army Post Offices with the Troops (_Figs._ 35-37). [Illustration: 35 36 37] There are also a number of types of Censor marks, not all necessarily military, and printed labels used in re-sealing opened letters (_Figs._ 38-40): [Illustration: 38 38A 39 40. 40A. 40B. 40C. 40D.] In this connection may also be mentioned the various marks used on letters sent by prisoners of war in charge of the British military authorities at home and abroad. An old type of prisoner-of-war mark dated 1800 is illustrated in _Fig._ 41, from "Pre-Victorian Postage Stamps and Franks" by Mr. G. A. Foster.[4] [Footnote 4: London. 1910: Charles Nissen & Co.] [Illustration: 41] The following are examples of the marks now being used (_Figs._ 42, 43): [Illustration: 42 43 42A. 43A.] It may be well to give a brief outline of the methods of the Army Postal Service, that its work may be better known and understood. In addressing letters to the troops it is important to give the full military particulars of the addressee, _viz_:--Regimental number, rank, name, squadron, battery or company, battalion, regiment (or other unit), staff appointment or department, and title of the Expeditionary Force. With these details set out clearly on the envelope, the work of the Army Postal Service is facilitated and the letter stands every chance of going through without delay. In France, as the postmarks already illustrated denote, the British Army Postal Service has several grades of post offices. The chief is the Base Post Office, the principal sorting establishment for all mail matter passing between our British Post Office and the Army Postal Service. The Base Office is quite a large concern and has a vast amount of clerical work to perform. In it letters are sorted, letters taking precedence over all other mail matter, after which the newspapers, and lastly the parcels are dealt with. Accounts of all the branch post offices are filed and the general routines and formulae of the Post Office at home are adhered to in detail. Letters, etc., for services, departments and units at the base are put into callers' boxes for delivery to the post orderlies. Those for more distant services and units are forwarded to the various grades of branch offices. At the Base Office one of the most complicated and difficult tasks is the re-direction of letters. Here are kept hospital lists, giving names of men away from their units in hospital, and these hospital rolls are revised weekly. Here also records have to be kept of the movements of the units, and these records are constantly in process of revision, and frequent communication is maintained with every branch office in the field.