In referencing periodicals, what's the difference between using p. or pp. for page numbers?

If a periodical includes a volume number, italicize it and then change to regular type and give the page range without pp.

If the periodical does not use volume numbers, include pp. before the page numbers so the reader will understand that the numbers refer to pagination.

Use p. if the source is a page or less long. For example, precede page numbers for newspaper articles and book chapters with p. or pp.

(adapted from the sixth edition of the APA Publication Manual, © 2010)