Periodicals Index Online (PIO) This link opens in a new windowPeriodicals Index Online is an electronic index to millions of articles published in over 6,400 periodicals in the humanities and social sciences. Periodicals Index Online was formerly known as Periodicals Contents Index (PCI). As of April 2010, Periodicals Index Online indexes over 15 million articles going as far back as 1665 and every article in each journal is indexed, from volume 1 issue 1 to recent times.