In those old Greek masks you always see outside theaters, Melpomene is the muse of tragedy wearing a sorrowful down-at-the-mouth mask, while Thalia, the muse of comedy, is always smiling. If a theater produced only tragedies, they’d shut the damn place down and run Melpomene out of town.

But in sports, because it’s real, fans can get trapped behind their self-imposed mask of mock tragedy for years, even decades, if their teams stay bad or embarrassing enough; the sad bill of fare seems to last for eternity.

There’s a new mask in Washington. Finally. Wear it, laugh behind it and prank with it until they pry it from your cold, dead, playoff-eliminated hands.

It’s not time for a parade, not by a long shot. But finally Washington fans can break out a smile, even a huge giddy grin if we like, one that will remain for days, weeks and probably for months.

They don’t give out the Stanley Cup for a first-round upset in the National Hockey League playoffs. Beleaguered owner Daniel Snyder didn’t just sell the Redskins. The Nationals still have 144 opportunities to make us forget a 14-4 start. And a five-game winning streak by the Wizards is like finding an empty canteen in the Gobi desert.

But for now, make way and give us some elbow room. For the moment, the Capitals are hailed as big-game players, Robert Griffin III is arriving any nanosecond and the Nationals have the second-best record in baseball. And Washington is a normal sports town — a theater with more than one mask.

When your city’s National Football League, NHL, National Basketball Association and Major League Baseball teams have been cruel punch lines for most of the last (rounding off), 20, 30, 30 and 80 years respectively, you’re entitled to celebrate solid good news that intimates the possibility of long-term change. It may not all pan out. But some of it will. (Even D.C. United, which hasn’t made Major League Soccer’s playoffs since 2007, appears to be shaking off its doldrums, currently in second place in the Eastern Conference. )

After being the undisputed No. 1 choking dogs of U.S. sport for decades, the Washington Capitals just beat the reigning Stanley Cup champion Bruins in overtime in Game 7 Wednesday night in Boston. “Defend” that!

Sure, there’s less pressure as underdogs and no boos from a home crowd. Forget that. What you’ll remember is the stunned, delicious moment when you saw the Caps dancing on their skate tips and you thought, “They actually did it! Or will they review the goal? Something’s still going to go wrong. They’re the Caps. No, it’s going to stand? It’s over!

And you can’t help but hope that what is “over” is not just this series but also a tradition of frustration so noxious that no franchise deserves it.

The NHL playoffs are so exciting because they are so capriciously unfair. Every sport has key postseason plays that turn, in part, on luck; but nothing like the NHL. That’s the league best and worst inherent feature. Almost every year, some talented but underachieving team fires its coach, gets healthy and adjusts to a new system as the playoffs approach, stumbles onto a “hot goalie,” enjoys its underdog role and goes to the Stanley Cup finals.

The story repeats so often it’s an NHL cliche, except for one twist of the knife: The Caps are never That Team. They may not be this year either, but for now, we can enjoy imagining it.

One night after the Capitals’ victory, the Redskins, as expected, drafted RGIII, a quarterback so talented that he will bring excitement instantly and victories eventually. (Probably.) D.C. is so nuts for the Redskins and so desperate for hope, anticipation for the NFL preseason will rival playoff madness in sane cities.

The history of good-to-great quarterbacks who are drafted either first or second overall is extremely consistent: Their first season is usually a bummer, a marginal improvement at best over the team’s previous record. But for the Peyton Mannings and Troy Aikmans, those rookie seasons mean nothing. It is the second year, or the third at the latest, when the truth shines.

After so much waiting, even the Redskins owner and fans — both historically characterized by premature optimism followed by impatience — will give Griffin (some) time to breathe and grow: The greater the talent, the longer the honeymoon.

Finally, as if to prove that anything is possible, and all of it arriving suddenly, all at once, right now, the Nats have raced to Washington’s best baseball start since 1932. While Redskins, Capitals and Wizards fans have had long hard times, D.C. baseball fans fall into the biblical suffering category: They’ve been waiting their whole lives, unless they remember the ’33 Series vividly.

But thanks to the Nationals’ fast start and Major League Baseball’s newly expanded playoff format, the dreams of Washington’s long-suffering baseball fans are not just conceivable; they’re realistic.

Everyone, including me, has misunderstood how (relatively) easy it will be to get into the new expanded MLB postseason, which will include 10 teams — three division champions and two wild-card entrants from each league. We’ve looked at the average number of wins that the hypothetical second wild-card teams actually had from 1995 through 2011, when eight teams made the playoffs. (It’s 88.9.) That’s nice information. But it’s misleading. The real standard is the least number of wins needed each year to get the last wild-card spot.

It’s like the old joke: If a bear is chasing you and a buddy, you don’t need to outrun the bear; you only have to outrun your friend. To win the second wild-card spot, you only have to be ahead of the third-best contender.

And, history says, that’s not very tough. Since ’95, an 86-76 record would have been good enough to make the new, expanded playoffs 13 times, plus nine more times when 86 wins would’ve put you in a tie. So, 86 wins would make you about a 50-50 postseason proposition. Also, just an 87-75 record would have won outright 21 of the 34 “second-wild-card” spot since ’95. That’s 57 percent, not including any tiebreakers.

Counterintuitive as it seems, if the Nats play .500 ball the rest of the season and finish 86-76, they’re about even money to make the playoffs. And if they just go 73-71, their chances would go above 60 percent.

Would an eight-game losing streak, as the Nats’ schedule toughens up, change all that? Well, yeah. But this is a daily newspaper. And, right now, here’s how it stands: The Nats have already done almost all the heavy lifting needed to become D.C.’s first postseason baseball team in 79 years.

The Caps may stumble, the Nats falter and RGIII be merely good. But the book on Washington pro sports is being rewritten before our eyes. The Caps don’t always choke. The owner doesn’t always blindside the Redskins. And Washington can have a contending baseball team.

How amusing. Thalia is back in town.

For Thomas Boswell’s previous columns, visit