A rivalry can be embedded within the fabric of fandom, graphed by geography, hemmed by history and fine-tuned by decades of disgust and mistrust.
Or, you know, one hot-headed pitcher can bean a batter for violating the "unwritten rules," the benches clear and -- voila! -- a new rivalry is born.
That's what makes a ranking of rivalries a difficult balance between history and the here and now. We had no way of knowing, going into the season, that one of the biggest beefs in MLB last year would be between the Royals and Blue Jays. It just sort of happened that way.
With all that in mind, this is an attempt to identify, with the help of the Hot Stove, the five best rivalries in the game right now. With all due respect to the strict city or state divides in such places as New York, Chicago and Missouri, recent postseason history involving the Cardinals and Dodgers, the aforementioned Royals-Blue Jays tiff, and even the hacking scandal featuring the Cardinals and Astros, my five revolve around intra-division drama, because that's what matters most.
5. Yankees-Red Sox
This rivalry is a mere shadow of what it once was, and, given the fact that the Red Sox have finished last in the American League East in three of the past four seasons and the Yankees finished well out of the running the lone year in that span in which Boston finished first (2013), there is a legitimate argument to strike it from the top five altogether.
But I just can't do it. There's too much history here and too much possibility that these two lovebirds are banging heads down the stretch again in 2016. Besides, you'd have to imagine David Ortiz's final season will induce some homages to 2003 and '04, if nothing else.

The Yanks reclaimed some respect with a 2015 built on bounce-back seasons from some of those old dudes in their lineup, while the Red Sox made some very serious offseason acquisitions to try to do the same. There's an interesting ninth-inning dynamic here now, with each club landing one of the best closers in the game (Aroldis Chapman to the Bronx, Craig Kimbrel to Boston). It will be interesting to see who fares better. And maybe the likes of Luis Severino, Greg Bird, Mookie Betts and Xander Bogaerts will garner some of the bad blood that once made this matchup so great.
For now, the best outward sign of continued strife came from Larry Lucchino, former president and CEO of the Red Sox and chairman for Triple-A Pawtucket, in a news release announcing the hiring of VP of sales Rob Crain, who previous worked at the Yankees' Triple-A affiliate Scranton/Wilkes-Barre.
"His defection from [the Yanks'] Minor League tentacles," the release said, "demonstrates hope for redemption for all."
Yep, still a rivalry, folks.
4. Mets-Nationals
Both of these clubs have their annual Interleague battles of regional appeal, with the Beltways Series and the Subway Series. And on that note, 2015 was probably the first time since the late 1980s that New York felt like a Mets town.
But again, nothing trumps a division duel, and this one has ratcheted up considerably thanks to a gentleman by the name of Yoenis Cespedes.

First, it was Cespedes' arrival at the Trade Deadline last summer that served as the biggest spark in the Mets outlasting the Nationals in a division race that everybody and their mother went into the year thinking would be won by Washington. Symbolically enough, Cespedes' first games with New York came during the three-game sweep of the Nats that permanently put the Mets in first place.
And then, as you know, Cespedes' long free-agent foray wound up centering around these East beasts -- the Nationals willing to go to five years, but the Mets taking advantage of Cespedes' desire to stay in the Big Apple with a three-year pact (and opt-out after 2016). So the Mets were able to retain a guy the Nats were hoping to steal, while Washington had already added longtime New York second baseman Daniel Murphy. Some interesting Hot Stove subplots that underscore the likelihood of these teams playing meaningful second-half games against each other again this year.
3. Rangers-Astros
You'll remember that not all Astros fans were on board with the idea of their club suddenly abandoning the Senior Circuit before the 2013 season. But the one benefit that even the fiercest detractors of that defection could agree with was the added meaning thrust upon the Lone Star Series. And in 2015, both of these clubs made a Texas-sized leap in the standings, meaning it was not only the first time since 2004 that both of these ballclubs were still in contention deep into the season but that they were directly opposing each other in the process.

No longer is Nolan Ryan's move from Arlington to Houston the most compelling plot point here (though it does satisfy as a subplot). The Astros were in first place for 139 days in the 2015 season, but the Rangers usurped that standing in the season's last two weeks, winning five of seven September meetings.
And these, of course, remain two legit contenders in a deep and compelling AL West for 2016 -- the Astros with their bullpen buoyed by Ken Giles and the Rangers with their rotation soon to be strengthened by the return of Yu Darvish.
2. Dodgers-Giants
Let me say, first of all, that I love the D-backs' direction, aggressively adding to what was a very good core and legitimately trying to vie with the National League West's perennial powers. The Dodgers partied in their pool a few years back, and that no doubt inspired D-backs president Derrick Hall, in a dead-on recent impersonation of Donald Trump, to jokingly call the Dodgers and their fans "vile, disgusting people." Maybe Arizona will make the West a legit three-headed monster this year.

But until the wins and losses flesh out that vision, there's no doubt Dodgers-Giants is the West's surest strife, and it satisfies both for its history and its present-day implications. These two teams have finished first and second in the NL West in three of the past four seasons. Over the past six seasons, Los Angeles has three division rings, while San Francisco has two division titles and three World Series rings.
So these are two of the sport's biggest successes, as well as two of its biggest spenders -- a point only amplified by the Giants' aggressive activity this winter.
Some would even paint the front offices here with a broad brush and call the Brian Sabean-led Giants "old school" and the Andrew Friedman-led Dodgers representative of the game's increasingly nerdy bent. I'm not sure either characterization is entirely fair or accurate, but hey, it functions well enough as a distinction in approach. For now, the upper hand here is clear: The Dodgers own the division, but the Giants own October.
1. Cardinals-Cubs
The Pirates are the division's other principal figure, but no one can deny that this is the NL Central's source of fiercest friction. Until 2015, it had been a rivalry that, save for a one-off moment like "The Sandberg Game" or the national narrative that was the Mark McGwire-Sammy Sosa home run record chase, existed only in the minds of the most passionate denizens of St. Louis and the Second City. The Cubs, frankly, did little to ensure the rivalry remained operative, and the two clubs had never even met in the postseason.
• Cubs-Cards rivalry heats up
Obviously, 2015 changed everything. In the regular season, the Cardinals and Cubs played 19 times. Seven of those games were decided by a run, and two more were decided by two runs. The Cards took the season series, but the Cubs -- foreshadowing the October result -- won six of the last nine. When they met up in the NL Championship Series, the Cubs played an unexpected round of Home Run Derby, with a memorable Kyle Schwarber blast landing atop the right-field video board at Wrigley, en route to a 3-1 victory.

Then, the defections, which added Hot Stove hype to the in-season struggle. First, the Cubs poached John Lackey. But the big one was Jason Heyward not only turning down a bigger offer from the Cardinals but miffing them in the process when he told Chicago reporters that the Cards' over-30 core of Yadier Molina, Matt Holliday and Adam Wainwright isn't going to be around much longer and that he preferred the long-term outlook for the Cubs' young group.
Them's fighting words, in the mind of a Cardinals club with a particularly impressive recent resume (five division titles and two World Series titles, as well as a third appearance, just in the past decade). And while the Cubs beat the Cards last October, they still have to prove they can outlast them in the 162-game schedule and claim a World Series crown of their own.