The New York Yankees debuted a new baseball bat at this weekend’s season-opening series against the Milwaukee Brewers, their second game of the season. The bat has been causing controversy, after it was responsible for some of the nine home runs the Yankees made at the game, tying a franchise record.

The Yankees, who won 20-9, began the game by getting three home runs off of the first three pitches thrown in the game. The Brewers starting pitcher, Nestor Cortes Jr., had his first three pitches struck out of the park by Aaron Judge, then Paul Goldschmidt, and finally, Cody Bellinger, leading to an early lead for the Yankees.

The impressive three home runs in a row were later followed by a fourth home run by Austin Wells in the same inning, making the game one for the record books: it’s the first time the Yankees have hit four home runs in one inning.

“What a performance,” Yankees manager Aaron Boone said to reporters after the game. “Kind of a weird, crazy game.”

During the contest, play by play announcer Mark Kay mentioned that some of the Yankees players were using newly designed bats. Two of the players using the bats hit home runs during the game.

What’s so special about the bats? 

According to former infielder Kevin Smith, the Yankees’ new bats were designed by Aaron Leanhardt, a former physicist turned baseball coach. Leanhardt, an ex-physics professor who earned his doctorate at the Massachusetts Institute of Technology, has been working for the team for over six years. He used to be the Yankees assistant hitting coach, before becoming an analyst. He recently moved to work for rival team the Miami Marlins.

Smith posted on X that the “torpedo” barrel bat was designed to decrease misses by hitters. The bat’s barrel is thicker and wider than a standard baseball bat. The goal is to bring more wood and mass to the part of the bat that makes most contact with the ball, according to Smith, so that the hitter is less likely to miss.

Not all of the players were using the new bats in the game, and some of the home runs were made with standard issue bats, but the record-tying home runs have led to questions about whether the bats are allowed in Major League Baseball.

Are the new bats allowed in Major League Baseball? 

New bats have to be approved by the MLB before being used in the field. This is according to MLB Rule 3.02, which states that “experimental” bats can’t be used “until the manufacturer has secured approval from Major League Baseball of his design and methods of manufacture.”

The rule also states that baseball bats must be one solid piece of wood that is shorter than 42 inches and cannot have a diameter greater than 2.61 inches.

A spokesperson for the MLB confirmed that the Yankees bats are in accordance with the rules and regulations for bats in the league, according to the Athletic. Tigers manager A.J. Hinch also acknowledged that there had been conversations about the new bat during the spring training season.

“The internet has a beautiful way of bringing things to be a big deal,” Hinch told reporters. “I hadn’t paid a ton of attention to it really until today.”

Innovation in bats have changed MLB 

This is not the first time a new bat has been introduced in Major League Baseball. Bats made out of white ash trees were used for decades. Then Joe Carter became the first notable baseball player to use a baseball bat made out of maple wood in the 1990s.

In 2001, Barry Bonds scored 73 home runs in a single season using a maple bat, helping that type of bat became more widely used in major league baseball. It was later discovered that Bonds had been possibly using steroids during this season, which may have contributed to his success.

The Yankees have also been at the center of another bat-related controversy in the MLB. In 1983, two home runs made by Kansas City Royals player George Brett were ruled out by an umpire after then Yankees manager Billy Martin noticed a large amount of pine tar on Brett’s bat.

The umpires ruled that the home run was made with an “illegally batted ball” because of a rule that states “a bat may not be covered by such a substance more than 18 inches [46 cm] from the tip of the handle.”

The ensuing protest led to a reversal of the decision, and Major League baseball amended the rules to say that if it is later determined that there is excessive pine tar on a baseball bat, the play cannot be nullified if the object is not raised prior to the bat being used.