But the regular-season series between the two clubs, now 87 games old, is just the current incarnation of a long sequence of Cubs/White Sox games which began in the earliest years of the 20th Century, most notably with the 1906 World Series, won four games to two by the underdog White Sox over a Cubs team that had steamrolled the National League with a 116-36 record.
From 1903 through 1942, the Cubs and White Sox played what became known as the "City Series" in many, though not all, Octobers. The teams were both in the World Series several times in that era (though they have not faced each other since that 1906 matchup), and when they weren't, they'd often play each other. All told, there were 26 of these series, dominated by the South Siders, who won 19 of them to the North Siders' six (the first one in 1903 ended in a tie). The White Sox won 91 games, the Cubs 60, and there were three ties in that pre-night-game era.
But those weren't the only matches before interleague play. After World War II, the teams played an annual in-season exhibition game called the "Boys Benefit Game", with proceeds to boys' baseball teams in Chicago. These matches alternated years in old Comiskey Park and Wrigley Field; the Cubs, though by far the worse team in the 1949-72 time frame these games were played, won the series 13-10. (They skipped 1950, since the White Sox hosted the All-Star Game that year.)
Two exhibition games between the teams were staged as warmups to the resumption of the strike-shortened 1981 season; on the evening of August 7 they played to a 0-0 tie at Comiskey Park, and the next afternoon the Cubs won 4-3 at Wrigley Field.
In 1985 the teams revived the annual charity event, now styled the "Crosstown/Windy City Classic". Without lights in Wrigley Field, the 1988 game was memorable: It went into long extra innings, and by its end, Cubs catcher Jim Sundberg and White Sox infielder Steve Lyons were pitching. It ended in a 7-7, 15-inning tie. That charity series ended after 1994; the White Sox won eight of the 10 games played, with the 1988 game and the final game in 1994 ending in ties. That '94 game could win you a bar bet with this trivia question: "What's the only major league baseball stadium Michael Jordan ever played in?" Jordan, who had signed to play baseball with the White Sox after his first retirement from the NBA, played right field for the White Sox at Wrigley Field on April 8, 1994. He went 2-for-4 with a single and a double and had two RBI in front of a near-capacity crowd on a 42-degree day.
The two teams finished up their pre-interleague competition with two exhibition tilts in 1995; as in '81, they were warmups after a labor stoppage, one at each team's park, both won by the White Sox.
And now they play in games that count every year. After the Cubs beat the White Sox June 26, 2009, the Cubs led the series 35 wins to 34; not only that, it was nearly even in runs scored after that game -- 344 for the White Sox, 341 for the Cubs.
Since then, it's been all White Sox -- they've won 14 of the 18 games played since, five by shutout, and the Sox have outscored the Cubs 72-41 in those games. With the White Sox hanging on to first place in the AL Central and the Cubs with the worst record in baseball, those numbers will probably get even more lopsided in the Chicago South Siders' favor.