Information scientists at RAND have had a continuing interest in the design and appropriate use of electronic mail systems. During the past decade, this interest has manifested itself in the design of the MH electronic mail system, widely distributed in many releases of the UNIX operating system. The original user's manual for MH was B.S. Borden, R. S. Gaines, and N. Z. Shapiro's The MH Message Handing System: User's Manual, The RAND Corporation, R-2367-AF, November 1979; guidelines for use of electronic mail systems were proposed in N. Z. Shapiro and R. H. Anderson's Toward an Ethics and Etiquette for Electronic Mail, The RAND Corporation, R-3283-NSF/RC, July 1985. Many MH users have exploited its power and adaptability without fully realizing the underlying source of that power. To data, the observations and principles underlying the design of MH have not been documented. This Note's purpose is to rectify that omission.