The National League against the American League.
It's a matchup that was exclusively reserved for the World Series -- that is, until 1997, when Major League Baseball injected Interleague Play into the regular season for the first time.
Interleague began as a couple of special weekends each summer when old city rivals finally got a chance to square off, and new rivalries were formed. Once the Astros moved over to the AL in 2013, Interleague Play became a year-long affair, giving us a much larger sample size going forward to ponder the age-old question: Which league is better?
After the first 20 seasons of this grand experiment, the preliminary results are in. Here are some things you should know about the first two decades of Interleague Play:

• When the Rangers hosted the Giants on June 12, 1997 (the first of four Interleague matchups that night), it was the first time in baseball history that an NL team had played an AL team in regular-season play. San Francisco landed the first punch for the Senior Circuit, winning 4-3.
• The AL holds the overall advantage, boasting a .529 win percentage and a +1,714 run-differential over the NL.
• After claiming four of the first seven season series through 2003, the NL has not prevailed in a single-season head-to-head since.
• The Yankees' .583 win percentage (211-151) make them the most successful team in Interleague Play, followed closely by the Red Sox (.581). In fact, AL teams hold the top nine Interleague records, with the Mets coming up as the first Senior Circuit team at No. 10.
• The longest win streak in Interleague Play is 13 games, shared by the Yankees (2003-04), then-Devil Rays (2004) and Marlins (2013-14). The Mets and Rays' four-game winning streaks are tied for the longest active runs going into 2017.
• The Reds and Padres have fared the worst with a .422 win percentage against the opposing league, followed by the Rockies, Phillies and Pirates.
• The Padres are also the unfortunate owners of the longest Interleague losing streak in history at 13 games from 2008-09. Arizona is looking to break its current MLB-high nine-game losing streak against AL foes.
• The apparent limit for Interleague contests -- so far -- is 17 innings. That record for longest Interleague game has been matched seven times, most recently when the Padres beat the Blue Jays, 4-3, in 17 frames on May 31, 2013.
• Max Scherzer's 20-strikeout effort against the Tigers this past May was notable for a lot of reasons, including the fact that it broke Corey Kluber's record (18) for most punchouts by a pitcher in an Interleague matchup.
• Five different pitchers have twirled no-hitters against teams from the opposite leagues, from David Cone's 1999 perfect game against the Expos to, most recently, Mike Fiers' no-hitter against the Dodgers in August 2015.
• The Phillies' Jamie Moyer beat three different AL teams after his 47th birthday in 2010 to cement his status as the oldest starting pitcher to win an Interleague game. The youngest is Mariners ace Félix Hernández, who beat the Padres at the age of 20 years, 43 days on May 21, 2006 -- despite giving up seven earned runs over six innings.

• No one has slugged four homers in an Interleague game yet, but 22 players have hit three. Red Sox teammates Mookie Betts (Aug. 14 vs. Arizona) and Hanley Ramirez (July 20 vs. San Francisco) each did so this past season.
• Though it took him three seasons to compile, former Twins outfielder Matt Lawton holds the longest Interleague hitting streak at 37 games, followed by Ichiro Suzuki at 31 games. Meanwhile, Tigers ace Justin Verlander must have seemed unbeatable to NL teams from 2010-14, when he won a record 14 straight decisions against Senior Circuit clubs. Interestingly, Verlander is winless in three decisions in the World Series.

• Only eight players have hit for the cycle in an Interleague game. Texas' Shin-Soo Choo was the latest to do so, finishing off his cycle with a ninth-inning triple in the Rangers' 9-0 victory over the Rockies on July 21, 2015. The batter directly after Choo in the lineup, Delino DeShields, finished just a homer shy of getting his own cycle that night.