The Publisher Item Identifier (PII) is a unique identifier used by a number of scientific journal publishers to identify documents.[1] It uses the pre-existing ISSN or ISBN of the publication in question, and adds a character for source publication type, an item number, and a check digit.
The system was adopted in 1996 by the American Chemical Society, the American Institute of Physics, the American Physical Society, Elsevier Science, and the IEEE.