According to the recordkeeping practice of the time, letters received were entered in registers, generally in chronological order by date of receipt, and numbered consecutively within each year. The letters are arranged to correspond with the arrangement of the numbered entries in the registers. The letters received are endorsed on the back or on a separate cover sheet with the name of the writer, the date of writing, the place of writing, the purport of the contents, the date received, and the file number assigned in the register. The file designation consists of the number assigned to the letter, the symbol "AGO," and the year the letter was received.
Because the letters were numbered and entered in the registers according to the dates of receipt rather than the dates of writing, and because there often was a considerable lapse of time between the date of writing and the receipt of a letter in the Adjutant General's Office, there is some overlapping of dates covered by the registers. This overlapping is reflected in the filing arrangement of the letters.
Some of the letters bear file citations other than those of the Adjutant General's Office. These citations indicate that at one time or another the letters had been filed in some other office. Occasionally drafts of orders, replies to letters received, and related working papers were filed with the letters received to which they relate. There are other variations in the arrangement pattern. (1) Some years after the original filing of these letters the Adjutant General's Office attempted to bring together files relating to certain subjects, which resulted in "consolidated files." For the 1871-80 period, these "consolidations" generally varied in quantity from a few to several thousand papers. Cross-reference slips appear throughout the file and indicate the file number under which the letters were consolidated. Following these introductory remarks is a list of 102 of the more significant consolidations and large files and of the rolls on which they have been reproduced. (2) The annual reports of War Department bureaus and geographical commands are filed together at the end of the letters for each year. After the list of consolidations mentioned in (1) above is a list of annual reports for the same period arranged by year and showing the numbers of the rolls of microfilm on which the reports have been reproduced. (3) In some instances errors were made by clerks when the letters were entered in the registers; sometimes registry numbers were omitted and at other times numbers were repeated. When the numbers were repeated, the Adjutant General's Office added "1/2" to the number of the second letter to distinguish it from the first one bearing the same number. Where this has not been done, the National Archives has added, in brackets, "No. 1" and "No. 2" to the file designation.
Some of the documents originally filed in this series are no longer in the series, others are incomplete, and some enclosures have been separated from their transmittal letters. Some of the registered communications were referred to other offices or agencies and eventually became incorporated in the files of those agencies. Others were removed and filed in different series of records of the Adjutant General's Office. Removals frequently were documented by means of cross-references. Some of the cross-reference slips give the name of the writer, the file number, the date of transfer, and the name of the office to which the records were transferred. Others simply give the file number and the name of the office or person to whom the records were transferred. Where a cross-reference slip or the register of letters received indicates a document was transferred to another series of records in the Adjutant General's Office or to another office, the National Archives has not searched the records in these series to locate the letter entered in the register.
Sometimes the Adjutant General's Office inserted a cross-reference slip bearing one of several special stamps to denote the removal of records, (1) A stamp signifying "To JAG, June 6, 1894," was used to document the transfer to the Judge Advocate General's Office of papers relating to the acquisition or sale of military reservations or public lands under the jurisdiction of the War Department. (2) A stamp stating "Insane Soldier; Papers to Regimental Records Division for file with personal papers; G. W. Pratt," was placed on the cross-reference slip whenever papers were removed relating to insane enlisted men of volunteer units of the Union Army. Many of these papers are now with the Union Army compiled service records. (3) A stamp reading "With records 'Provost Fund’ Archives Div., R. & P.0., Feb. 24,  04," was used to denote the transfer to the Records and Pension Office of records relating to the provost fund, which was composed of unexpended balances in the hands of provost marshals at the end of the Civil War and disbursed for various purposes under the order of the Secretary of War. The records that were transferred were added either to a series of letters received or to a series of vouchers relating to the fund. Both series form part of a group of records collectively known as the "Provost Fund Records."
In the same record group are other related records. Available on microfilm are Letters Sent by the Office of the Adjutant General (Main Series), 1800-90 (M565); Letters Received by the Office of the Adjutant General, 1805-21 (M566); Letters Received by the Office of the Adjutant General (Main Series), 1822-60 (M567); Letters Received by the Office of the Adjutant General (Main Series), 1861-70 (M6l9); Letters Received by the Office of the Adjutant General (Main Series), 1881-89 (M689); Registers of Letters Received by the Office of the Adjutant General (Main Series), 1812-89 (M711); and Indexes to Letters Received by the Office of the Adjutant General (Main Series), 1846, 1861-89 (MJ25).