Twins General Manager Terry Ryan was on the committee that developed baseball's new replay rule, which was unanimously approved by the 30 major league teams on Thursday, but he makes no claims that the system is perfect. "We know over time it's going to be tweaked, that things will come up that we didn't anticipate," Ryan said. "But we've got to start somewhere, and this is a good start."

The new rule starts with managers, who will have the right to challenge controversial calls beginning this season, and umpires watching replays in New York, who will decide whether to uphold them. Managers will be allowed to challenge one call per game and will be granted a second challenge if their first is upheld. But no manager will be allowed more than two challenges in a game, and arguing over a replay official's ruling will not be allowed.

It's a major expansion of replay in Major League Baseball, a sport which had limited the technology to home run calls until now. The new system will be tested during several exhibition games this spring, then go into effect on Opening Day.

"We're still going to have some calls that are missed, that's unavoidable. But the rule was designed to make sure a bad call doesn't decide a game, that we get as much right as we can," Ryan said. "This rule covers about 90 percent of the plays."

Balls and strikes cannot be challenged, but virtually all other controversial calls can (see box). The rule specifically creates an exception for the so-called "neighborhood play," a force at second base during a double play, which will not be reviewable.

One other exception: The current home run replay rule, in which umpires determine whether to review their call, remains in effect. Managers can request a review, but not use a challenge on those calls.

MLB has created a "command center" in New York where a rotating crew of major league umpires will be stationed, with access to replays from all stadium cameras, including some not shown on TV broadcasts. Each umpire will be responsible for reviewing plays from no more than two games. When a play is challenged, the crew chief on the field will contact the replay official, using a headset on the field, and the official will determine whether the call was correct and, if the call was overturned, where baserunners should be appropriately placed.

If a manager has already used his challenge, umpires can institute a replay review on their own, from the seventh inning on.

Teams will be allowed to station someone in the clubhouse to watch the game on TV, and communicate with the manager on whether to challenge a call. No TVs will be allowed in dugouts.

One other effect of the new rule: MLB has ended its policy of preventing replays of close plays from being shown on stadium scoreboards. Teams can now show any replay.

Major League Baseball owners and the players' union remain at work on drafting a rule that would ban home-plate collisions.