Umpires will be getting a third choice on potential home runs down the line: fair, foul, or replay.

Umpires will be allowed to check video on home run calls starting Thursday after Major League Baseball, guardian of America’s most traditional sport, reversed its decades-long opposition to instant replay.

“Every team can go home and sleep better at night if they know the call was right,” Oakland designated hitter Frank Thomas said.

Baseball commissioner Bud Selig, who described himself as “old fashioned” when he made the announcement Tuesday, softened his opposition following a rash of blown calls this year.

For now, video will be used only on so-called “boundary calls,” such as determining whether fly balls went over the fence, whether potential home runs were fair or foul and whether there was fan interference on potential home runs.

“Any time you try to change something in baseball, it’s both emotional and difficult,” Selig said. “There’s been some concern that, well, if you start here, look what it’s going to lead to. Not as long as I’m the commissioner.”

Replay will go into use with three series scheduled to open Thursday: Philadelphia at the Chicago Cubs, Minnesota at Oakland and Texas at the Los Angeles Angels. For other games, replays will be available to umpires starting Friday.

Cubs manager Lou Piniella wondered whether a team could challenge a call.

“I’d love to be able to throw a red hankie or a green hankie. Imagine being able to throw something on the field and not be ejected,” he said. “I shouldn’t say it’s not going to work, but this could turn into a little bit of a fiasco initially.”

The NFL first used replay to aid officials in 1986, the NHL in 1991 and the NBA in 2002. Even at stuffy old Wimbledon, technology has been used on line calls since 2006. Replay equipment to help determine calls was in place at this year’s Little League World Series.