Flying a flag over Wrigley Field to denote wins and losses is a longstanding tradition.
Currently, the Win Flag is composed of a large blue letter "W" on a white background while the Loss Flag is a large white letter "L" on a blue background.
Early in the 21st century, the phrase "White flag time at Wrigley!"
was used to explain that the Cubs had just won that day's home game.
More recently, at least as early as the 2015 season, the Cubs have used the phrase "Fly the W!";
an accompanying social media hashtag "#FlyTheW" has been heavily promoted by the team and its fans.
The flag is raised by a scoreboard crew member immediately after the completion of a game, and in the case of a doubleheader split, both flags are flown.

It is customary to fly flags from sunrise until sunset, unless they are directly illuminated.
 At the beginning of each day, the cross-shaped "masthead" atop the center field scoreboard displays the American flag at the top, and three strands of flags bearing the colors and nicknames of each of the National League clubs, one strand per Division in order of that day's standings.
Immediately after the game, one of the strands of flags is lowered, and either the W or the L-lettered flag is raised in its place.
The W-lettered flag is raised on the left field side of the board, above the blue light that also indicates a win.
The L-lettered flag is raised on the right field side of the board, above the white light that indicates a loss.
Once the Win or Loss Flag has been raised, the other flags are lowered and also stowed away inside the scoreboard.

The symbolism of the flag now serves a wide variety of purposes.
 It is commonly reproduced in 3-by-5-foot (0.91 by 1.52 m) dimensions, and car flag versions are also sold.
 The flag is also available in 2-by-3-foot (0.61 by 0.91 m) dimension versions.
 The flag has become a celebratory symbol for Chicago Cubs fans who purchase reproductions and take them to games at home and on the road to show support.
The fans wave them after victories at Wrigley Field, and they hang them in the stands during playoff series.
 Some fans fly a win flag in place of or in addition to the Flag of the United States on days when the Cubs win.
Since 1998, the flags have become ever more popular, with demand reaching a level where they are sold at Wrigley Field.
In 2007, with the Cubs in playoff contention for the first time in 3 years, the Cubs' victory tune, Go Cubs Go!
became more popular among the fans.
Due to the song's growing popularity, after wins at home, Cubs TV broadcasters Len Kasper and Bob Brenly would have their microphones shut off, while the camera pans around the stadium to view the jubilant fans as Go Cubs Go!
plays in the background.
After fans discovered this, they started purchasing "W" flags and waving them after wins at home as an attempt to get on TV.
However, in seasons when the Cubs are performing poorly the tradition has led to parody and satire, such as stories of the Cubs retiring the win flag due to signs of neglect.

While not having the popularity of the Win Flag, the Loss Flag has become increasingly popular with clubs who traditionally have rivalries with the Cubs, such as the St. Louis Cardinals, Milwaukee Brewers, or Chicago White Sox.
Fans of those teams wave the blue flag after their team defeats the Cubs at Busch Stadium or Guaranteed Rate Field.
However, because of the enormous fan base that the Cubs hold, it isn't uncommon to see fans at games on the road waving their "W" flags during Cubs wins on the road.

The flag is known by many names, with none being more prevalent than the rest, and may often be referred to by more than one name by the same manufacturer.
 "Chicago Cubs W Win Flag" is the name used on Amazon.com, however the company also refers to the flag as Chicago Cubs 'W' Banner Flag in their marketplace section.
 CubWorld.com also refers to the flag by the name Chicago Cubs 'W' Banner Flag.
 Other retailers use the names Cubs Win W Flag, Chicago Cubs Win 3' x 5' Flag, Cubs W Flag,  or (Chicago Cubs) W Flag in the case of Sports Fan Wareh
Flying a flag over Wrigley Field to denote wins and losses is a longstanding tradition. Currently, the Win Flag is composed of a large blue letter "W" on a white background while the Loss Flag is a large white letter "L" on a blue background. Early in the 21st century, the phrase "White flag time at Wrigley!" was used to explain that the Cubs had just won that day's home game. More recently, at least as early as the 2015 season, the Cubs have used the phrase "Fly the W!"; an accompanying social media hashtag "#FlyTheW" has been heavily promoted by the team and its fans. The flag is raised by a scoreboard crew member immediately after the completion of a game, and in the case of a doubleheader split, both flags are flown.
It is customary to fly flags from sunrise until sunset, unless they are directly illuminated.  At the beginning of each day, the cross-shaped "masthead" atop the center field scoreboard displays the American flag at the top, and three strands of flags bearing the colors and nicknames of each of the National League clubs, one strand per Division in order of that day's standings. Immediately after the game, one of the strands of flags is lowered, and either the W or the L-lettered flag is raised in its place. The W-lettered flag is raised on the left field side of the board, above the blue light that also indicates a win. The L-lettered flag is raised on the right field side of the board, above the white light that indicates a loss. Once the Win or Loss Flag has been raised, the other flags are lowered and also stowed away inside the scoreboard.
The symbolism of the flag now serves a wide variety of purposes.  It is commonly reproduced in 3-by-5-foot (0.91 by 1.52 m) dimensions, and car flag versions are also sold.  The flag is also available in 2-by-3-foot (0.61 by 0.91 m) dimension versions.  The flag has become a celebratory symbol for Chicago Cubs fans who purchase reproductions and take them to games at home and on the road to show support. The fans wave them after victories at Wrigley Field, and they hang them in the stands during playoff series.  Some fans fly a win flag in place of or in addition to the Flag of the United States on days when the Cubs win. Since 1998, the flags have become ever more popular, with demand reaching a level where they are sold at Wrigley Field. In 2007, with the Cubs in playoff contention for the first time in 3 years, the Cubs' victory tune, Go Cubs Go! became more popular among the fans. Due to the song's growing popularity, after wins at home, Cubs TV broadcasters Len Kasper and Bob Brenly would have their microphones shut off, while the camera pans around the stadium to view the jubilant fans as Go Cubs Go! plays in the background. After fans discovered this, they started purchasing "W" flags and waving them after wins at home as an attempt to get on TV. However, in seasons when the Cubs are performing poorly the tradition has led to parody and satire, such as stories of the Cubs retiring the win flag due to signs of neglect.
While not having the popularity of the Win Flag, the Loss Flag has become increasingly popular with clubs who traditionally have rivalries with the Cubs, such as the St. Louis Cardinals, Milwaukee Brewers, or Chicago White Sox. Fans of those teams wave the blue flag after their team defeats the Cubs at Busch Stadium or Guaranteed Rate Field. However, because of the enormous fan base that the Cubs hold, it isn't uncommon to see fans at games on the road waving their "W" flags during Cubs wins on the road.
The flag is known by many names, with none being more prevalent than the rest, and may often be referred to by more than one name by the same manufacturer.  "Chicago Cubs W Win Flag" is the name used on Amazon.com, however the company also refers to the flag as Chicago Cubs 'W' Banner Flag in their marketplace section.  CubWorld.com also refers to the flag by the name Chicago Cubs 'W' Banner Flag.  Other retailers use the names Cubs Win W Flag, Chicago Cubs Win 3' x 5' Flag, Cubs W Flag,  or (Chicago Cubs) W Flag in the case of Sports Fan Wareh
Cubs Win Flag