Q. Why do you use a colon to separate page numbers in a journal article citation but a comma to separate page numbers in most everything else? It seems completely nonsensical.
A. You’re right, a comma might make more sense. Somewhere along the line, however, the idea that a colon should come between a volume and page number morphed into Chicago’s current journal citation style. The following example, from the Botanical Gazette, appeared in the first ten editions of the Manual (the 1 in parentheses was used to mark the first of two examples by the same author):
LIVINGSTON, B. E., (1) On the nature of the stimulus which causes the change in form of polymorphic green algae. BOT. GAZ. 30:289–317. 1900.
That was offered not as an example of Chicago style but of Gazette style. Today, that same article can be found on the website of the International Journal of Plant Sciences (which continues the Gazette, a journal that got its start in 1876) and would be cited in modern Chicago style as follows:
Livingston, Burton Edward. “On the Nature of the Stimulus Which Causes the Change of Form in Polymorphic Green Algæ.” Botanical Gazette 30, no. 5 (1900): 289–317. https://doi.org/10.1086/328048.
An issue number and date now intervene between volume number and page range, but that colon nonetheless became Chicago style for citing journal articles, starting with the 12th edition (published in 1969). That was also the first edition to prefer Arabic rather than Roman numerals for volume numbers (the Gazette example was an exception to this rule in earlier editions), which may have factored into the decision. A colon remains Chicago style for referring to a volume and page number alone, as in “2:37” (for vol. 2, p. 37); see CMOS 14.74. Older Chicago style would have called for “II, 37” (with a Roman numeral and a comma, as in the 10th ed., ¶ 253).
The editors of the 12th edition could have specified a comma whenever a year or other number intervenes between volume and page numbers (as is the case for most journal article citations), but they didn’t. More than fifty years later, our editors have come to think of that colon as a helpful little sign that says journal article.