Baseball scoring rules are sometimes confusing even for baseball insiders. They can be downright difficult to understand for the casual baseball fan and are practically Greek to a baseball outsider. The rules to determine the game's winning pitcher are confusing because there are so many variables that can affect the outcome of who gets credited with the win.
The winning pitcher must pitch for the winning team. The starting pitcher must pitch at least five full innings to be considered for a win. If only five innings are played on defense (usually due to inclement weather), the starting pitcher must pitch at least four innings to be considered for the win. A relief pitcher does not have an innings requirement to earn a win.
Keep the Lead
Assuming he has met the innings requirement, when a starting pitcher leaves the game and his team is ahead, he is eligible for the win. If his team holds on for the win without relinquishing the lead at any point in the duration of the game, the starting pitcher will earn the win.
As soon as the opposing side ties the ball game or takes the lead, the pitcher who left the game with the lead is no longer eligible to earn the win.
When a starter has left the game and is ineligible for the win (or later becomes ineligible due to the opposing team tying the score), a relief pitcher becomes eligible for the win. The relief pitcher who is in the game when his team takes the lead will earn the win (assuming the opposing team does not tie or take the lead). If a relief pitcher is ineffective in his outing, yet his team still takes the lead before a new pitcher is brought in, the official scorer will award the win to the relief pitcher who was most effective in the team's victory.