“From 1903 until 1942, excluding the years one team or the other won the pennant, the Cubs and White Sox played each other in a best-of-seven postseason City Series. These series were officially sanctioned by the National Baseball Commission and, later, the Commissioner of Baseball. “Report
from wikipedia
https://en.m.wikipedia.org/wiki/Cubs%E2%80%93White_Sox_rivalry
“From 1903 until 1942, excluding the years one team or the other won the pennant, the Cubs and White Sox played each other in a best-of-seven postseason City Series. These series were officially sanctioned by the National Baseball Commission and, later, the Commissioner of Baseball. “Report
more on the City Series.
https://sabr.org/journal/article/history-of-the-chicago-city-series/Report