NEW YORK — 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.

Fan interference has been a big issue in baseball, with almost constant debate since Jeffrey Maier reached over the wall and gave Derek Jeter a home run during the 1996 AL championship series. Many blown calls have occurred at newer ballparks, where fans are closer to the field and have the ability to reach over fences.

"Everybody was tired of hearing about it. So they finally had to step up and do something to make a change," Angels pitcher Jered Weaver said.

Detroit pitcher Kenny Rogers called the decision "a slap in the face of umpires that have been here for a long time" and said the decision might have been made because Alex Rodriguez lost a home run on a blown call May 21.

"It overshot the mark by far just because, what, in a Yankee game someone didn't get a homer? Please. It's happened thousands of times," Rogers said. "That's part of the game. It's the beauty of the game. Mistakes are made."

Baseball general managers voted 25-5 last November to recommend use of the technology, and baseball's lawyers spent recent weeks finalizing agreements with the unions for umpires and for players.

"I find it very strange that, with 30 games to go in the season, that they would start it now. I find that very peculiar," Baltimore Orioles manager Dave Trembley said. "If they wanted it so bad, what took them so long to get it going and why wait until this particular point in time?"

Baseball officials wanted to avoid having a situation in the postseason where fans with access to televisions and viewers at home knew what the correct call was but the umpires on the field did not.

"Some people thought that we ought to wait until the postseason," Selig said. "I'd rather go into the postseason knowing that we've already used it."

Leaving the dugout to argue a call following a replay will result in an automatic ejection. Replays of the boundary calls will not be shown on stadium video boards, MLB executive vice president for baseball operations Jimmie Lee Solomon said.

MLB said replay delays will be offset by fewer arguments.

"So if the game is held up for a couple of minutes a couple of times a year, I think that's OK," New York Yankees pitcher Mike Mussina said. "It's certainly not going to be seen as often as it is in the NFL."

Umpire Gary Cederstrom said his crew had a training session Tuesday at Yankee Stadium.

"We talked to the technicians and he explained what they're going to be doing," he said. "We just basically did a dry run."