Modern Chess Openings (usually called MCO) is a reference book on chess openings, first published in 1911 by the British players Richard Clewin Griffith (1872–1955) and John Herbert White (1880–1920). It has since been through fifteen editions.
MCO was popular with English-speaking players and has continued to be updated throughout the 20th century and into the 21st, with fifteen editions from 1911 through 2008. Early editions were small enough to fit in a pocket (the first edition was 190 pages), but later editions grew and the fifteenth and most recent version is 768 pages.
In the first episode, Shaibel gives Beth a copy of the 7th (1946) edition of MCO, which Jolene steals out of jealousy when Beth is adopted, and returns to her years later. Beth later buys a copy of the 9th (1957) edition.