The Correspondence series, 1.5 linear feet, is arranged in alphabetical order. The types of correspondence range from personal letters, letters of recommendation, opinions on law matters or academia, and personal or financial business letters. For subjects (either individuals or institutions) with a large amount of correspondence, their materials were separated out into individual folders. Examples of these subjects include Allen's brother William, his brother Clark, various departmental units at the University of Michigan (the Law School, University Press, etc.), his close friend and colleague Norval Morris, and personal correspondence. Allen indicates on the upper right hand of most correspondence pages the name of the individual that the document pertains to, but his schema of doing this is at times inconsistent. Although the majority of the papers are organized by last name, sometimes they will be organized by the organization he was writing to instead of the individual to whom the letter was actually addressed (examples being the Harvard Law Review or the University of Chicago Press). In these cases the Harvard Law Review correspondence would be placed in the "H" folder, and the University of Chicago correspondence in the "C" folder. When there was a large number of documents pertaining to one individual or unit, care was taken to place the documents in reverse chronological order (in most cases, this was the original order that Allen used).
The personal correspondence folder includes information regarding the publishing of Allen's works from various university presses (fax receipts, contracts, etc.), any documents containing Allen's social security number, and letters and cards of a personal nature. Although documents of this kind are also in other correspondence folders, the respect of the original order has been maintained.
Common correspondents include:
- Allen’s family: Clark Allen and William Allen.
- Faculty colleagues at the University of Michigan: Lee C. Bollinger, Terrence Sandalow, Theodore St. Antoine, Thomas E. Sunderland, and Allan F. Smith.
- Colleagues at other law schools or professional organizations: Paul D. Carrington, Albert W. Alschuler, Dick Julin, Sanford Kadish, James Marsh, Norval Morris, Helen Silving, and Frank Zimring.
- Institutions:Harvard Law Review, departments within the University of Michigan (University Press, Administrative Affairs, Law School), Northwestern University, the Rockefeller Foundation, and the Salzburg Seminar.
- Others: Supreme Court Associate Justice Byron R. White, Walter O. Weyrauch, and Austin Wright.