What is a middle-digit system?

• A. Middle group digits become primary indexing unit; first group secondary; terminal group tertiary.
• B. Last digit is primary.
• C. First digit is primary.
• D. It uses letters as primary unit.

Answer: (A)

Middle-digit filing is an indexing approach where a record’s number is divided into three parts, and the middle part is used as the primary key. The digits in the first part become the secondary key, and the digits in the last part are the tertiary key. This setup helps distribute records more evenly and makes filing and retrieval more efficient, especially as new records are added, since changing or inserting numbers won’t require reworking the primary group for many files. For example, with a three-part number like 12-34-5678, the middle digits 34 are the primary index, 12 is the secondary index, and 5678 is the tertiary index. Using the last or first digit as the primary would lead to clustering and less efficient lookup, and using letters would describe a different system altogether.