(Chicago Manual of Style 14.232)
Title, edition (date), s.v. "Entry Title."
1. The New Encyclopedia Britannica, 15th ed. (2007), s.v. “Neanderthal.”
2. The New Encyclopedia Britannica, 15th ed. (2007), s.vv. “Pony Express,” “Oregon Trail.”
Any edition other than the first must be specified.
Include _ ed. after title if needed.
In a work that is arranged alphabetically by entry include s.v. or s.vv. (plural) followed by the names of the entry/entries instead of the page number.
Well-known major reference books are normally only cited in the notes.