The Cubs' then-owner Philip K. Wrigley ordered the reconstruction of the bleachers and the building of the manually operated scoreboard in 1937.
The scoreboard's "masthead" was in place by 1938, displaying the American flag at the top and flags representing the eight clubs of the National League on the left and right strands of the masthead, four apiece.
In the 1940s, the practice of raising the Win/Loss flags was begun.

The flags were originally a white "W" on a blue flag and a blue "L" on a white flag.
This matched what was then the color scheme of the Cubs "team flags" that flew on the foul poles, and the many little "W.F." flags that flew on the grandstand roof (all of them serving to give the players a sense of the wind speed and direction).
Ernie Banks was the first Cub to have his number retired on August 22, 1982, and his number 14 has been flown in blue on a white flag (echoing the pattern of a Cubs home uniform shirt) on the left field foul pole ever since.
On August 13, 1987, Billy Williams had his number retired and it flew with the same color scheme, on the right field foul pole.
The scheme of the Cubs Win flag was reversed in the early 1980s.
The change in the flag color scheme matched the Win flag color with the honored colors of the retired number flags, as the white-on-blue Cubs "team flag" was retired.
Subsequent retired Cubs numbers for Ron Santo, Ryne Sandberg and Ferguson Jenkins/Greg Maddux also match this color scheme.

In addition to the flags, Wrigley authorized Bill Veeck, the leader of the bleacher reconstruction project, to add colored lights on the crossbar of the masthead so that the Chicago 'L' passengers would be able to see the outcome of Cubs home games after sunset.
In his autobiography, Veeck remembered the colors as green for a win and red for a loss.
Hartell's book acknowledges Veeck's comments, but reports that contemporary newspapers stated that the colors of the lights were blue and white, as they are now.
Eventually the original lights on the masthead were replaced by lights directly on the top of scoreboard, still visible from outside Wrigley field.
The lights complement the flags by helping night time passersby learn the result of that day's Cubs game at a glance.
The current blue and white Win/Loss lights were added atop the scoreboard in 1978, while the "Wintrust W" light on the left-field video board was implemented in 2015.

On April 23, 2008 the Cubs won for the 10,000th time in the history of the franchise on the road against the Colorado Rockies.
Like the 9,000th win, the 10,000th win came in a 7–6 victory at Coors Field.
The Cubs were the second franchise to achieve 10,000 regular season wins; the San Francisco Giants/New York Giants were the first.
The franchise flew a special 10,000th win flag along with a Cubs Win flag to commemorate the landmark achievement for the franchise.
This was unusual not just for the 10,000th win, but also for flying the flag in reference to a road game.
The special flag flew from the afternoon of April 24, 2008 until immediately prior to the next home game on April 29, 2008 at 7:05pm against the Milwaukee Brewers.
After the flag was taken down, it was autographed by all members of the 2008 Chicago Cubs and auctioned off for charity.
The Cubs' then-owner Philip K. Wrigley ordered the reconstruction of the bleachers and the building of the manually operated scoreboard in 1937. The scoreboard's "masthead" was in place by 1938, displaying the American flag at the top and flags representing the eight clubs of the National League on the left and right strands of the masthead, four apiece. In the 1940s, the practice of raising the Win/Loss flags was begun.
The flags were originally a white "W" on a blue flag and a blue "L" on a white flag. This matched what was then the color scheme of the Cubs "team flags" that flew on the foul poles, and the many little "W.F." flags that flew on the grandstand roof (all of them serving to give the players a sense of the wind speed and direction). Ernie Banks was the first Cub to have his number retired on August 22, 1982, and his number 14 has been flown in blue on a white flag (echoing the pattern of a Cubs home uniform shirt) on the left field foul pole ever since. On August 13, 1987, Billy Williams had his number retired and it flew with the same color scheme, on the right field foul pole. The scheme of the Cubs Win flag was reversed in the early 1980s. The change in the flag color scheme matched the Win flag color with the honored colors of the retired number flags, as the white-on-blue Cubs "team flag" was retired. Subsequent retired Cubs numbers for Ron Santo, Ryne Sandberg and Ferguson Jenkins/Greg Maddux also match this color scheme.
In addition to the flags, Wrigley authorized Bill Veeck, the leader of the bleacher reconstruction project, to add colored lights on the crossbar of the masthead so that the Chicago 'L' passengers would be able to see the outcome of Cubs home games after sunset. In his autobiography, Veeck remembered the colors as green for a win and red for a loss. Hartell's book acknowledges Veeck's comments, but reports that contemporary newspapers stated that the colors of the lights were blue and white, as they are now. Eventually the original lights on the masthead were replaced by lights directly on the top of scoreboard, still visible from outside Wrigley field. The lights complement the flags by helping night time passersby learn the result of that day's Cubs game at a glance. The current blue and white Win/Loss lights were added atop the scoreboard in 1978, while the "Wintrust W" light on the left-field video board was implemented in 2015.
On April 23, 2008 the Cubs won for the 10,000th time in the history of the franchise on the road against the Colorado Rockies. Like the 9,000th win, the 10,000th win came in a 7–6 victory at Coors Field. The Cubs were the second franchise to achieve 10,000 regular season wins; the San Francisco Giants/New York Giants were the first. The franchise flew a special 10,000th win flag along with a Cubs Win flag to commemorate the landmark achievement for the franchise. This was unusual not just for the 10,000th win, but also for flying the flag in reference to a road game. The special flag flew from the afternoon of April 24, 2008 until immediately prior to the next home game on April 29, 2008 at 7:05pm against the Milwaukee Brewers. After the flag was taken down, it was autographed by all members of the 2008 Chicago Cubs and auctioned off for charity.
Cubs Win Flag