Baseball: Rules for pitching lefty or righty

[Marley23]

You can find some details in this thread from 2008, which was inspired by this story about Venditte facing a switch hitter. The two of them swapped sides over and over again. Quoting from there:

@Obeseus

[Greg] Harris switch-pitched on September 28, 1995, the penultimate game of his career. […]
MLB came out with guidelines (not actual playing rules) for umpires to use in case Harris were to switch-pitch. Those guidelines specify that the pitcher must pitch with the same arm for each at bat. (A batter may switch boxes after every pitch up until the pitcher is set to pitch.)