David Krejci and Milan Lucic, as well as Patrice Bergeron and Brad Marchand, are each entering their fifth seasons as linemates.

BOSTON — Chemistry is a hazy term, a nebulous catch-all that hockey people like to use.

Coaches want to see which players have chemistry and teammates try to develop it in practice. Some teammates have chemistry immediately, while others can be perfect together on paper but, for whatever reason, don't mesh on the ice.

For those that have it, though, it can be like being an old married couple. You know the other person perhaps better than you know yourself, where they’re going to be and how you can connect with them. And those times that would be uncomfortable with someone else, well, they’re just not.

“He’s the kind of guy,” David Krejci says of long-time linemate Milan Lucic, “that I’d say when you’re driving the car, and there’s that awkward silence — it’s not awkward. It’s just natural. You don’t have to talk or you don’t always have to talk, it just feels the same. It’s such a good, friendly feeling.

“He’s become such a good friend of mine and I’m really happy I can be playing with him on the same line.”

Krejci and Lucic have been linemates since the start of the 2010-11 season. Brad Marchand and Patrice Bergeron started sharing shifts later that season and have remained linemates since.

The left wing and center on both of the Bruins’ top lines have been held together by magnets. For each duo, this will be their fifth season together.

As the Bruins sort through a jumble in their bottom two lines to start the season, they will look to their top two lines more than ever. And really, that means looking to Lucic and Krejci, and Bergeron and Marchand.

The Bruins build their lines in pairs. This is the third straight season there’s been a different right wing with Lucic and Krejci, after the departures of Nathan Horton and Jarome Iginla. Loui Eriksson is expected to fill those lanes this season. Marchand and Bergeron have cycled through Mark Recchi, Tyler Seguin, briefly Eriksson, and now for a second straight season Reilly Smith.

Spoked-B management is OK with swapping out the right wing — as long as the other two are together.

“I go back to [Stanley Cup-winning coach] Jacques Lemaire, who I have a good friendship with,” coach Claude Julien mused, “and I remember when he was coaching, he always had these pairs that he would go with. He would always change that third guy, and that third guy could be interchangeable, but he tried to keep that pair together.

“He really felt that certain pairs should have good chemistry. I’ve always held that in the back of my mind because it does make sense.”

Best of all?

Marchand and Bergeron played their first game together Dec. 30, 2011 at Philips Arena against the old Atlanta Thrashers.

Marchand had climbed his way up the lineup after starting the season on the fourth line, and two nights after Marchand scored a goal at Tampa Bay, Julien started the rookie with Bergeron and Recchi. Bergeron scored that night on a breakaway, although Marchand wasn't on the ice (Daniel Paille hadn’t finished his shift yet as the puck transitioned quickly).

“I remember Brad was a younger guy coming in,” Bergeron said. “He had lots of energy and he definitely wanted to make a statement and stay. I just wanted to play off the energy he had and we did establish some chemistry pretty quickly. It went from there.”

And from there, they've become perhaps the best two-way line in the league.

Julien uses the two-time Selke Trophy-winning Bergeron to the maximum — Bergeron excels in all zones, and the coach asks him to do just that. Last season, Bergeron started 46.0 percent of his shifts in the offensive zone, the second-lowest rate among Boston forwards. He’ll often be called upon as a faceoff specialist for another line; Bergeron will win the draw, gain possession for the team, then retreat to the bench until his regular shift.

So it’s no surprise that when Bergeron plays without Marchand, the Bruins score just 46.7 percent of the time. But when he plays with Marchand, the Bruins literally doubled up the opposition last season (66.7 percent of goals scored when the two are on the ice were scored by the Bruins, per stats.hockeyanalysis.com).

“I think it’s been five years in the making, every year a little bit more,” Marchand said of their chemistry. “We seem to be able to read off each other a little more. It took a couple months to get some plays down and a couple years where we have certain plays that we want to perfect. I think a lot of our goals are pretty similar. I think we found a few plays that work and we want to use those.”

One of those plays is Marchand's ‘stop short’ move. Marchand is the left wing, but is just as capable of carrying the puck through the neutral zone and over the blue line as his center is. Having a second puck-handler allows the line to nearly always get clean entries into the offensive zone — watch how rarely they dump and chase — a huge factor in their mammoth possession numbers.

One of Marchand’s favorite moves is to come down the left side with speed. When the defender is busy back-tracking, Marchand pulls up with the puck, leaving the defender falling back, and giving himself time and space to evaluate his options. He can pass to a forward streaking to the net, or find a defenseman trailing the play by the blue line.

After four years of playing together, Bergeron knows what to do when he sees Marchand coming to a stop. Marchand used the move in Game 2 of the Bruins' playoff series with Montreal last spring. As Marchand pulled up, Dougie Hamilton was wide open as he approached the blue line.

Bergeron immediately began positioning himself to screen goalie Carey Price, sucking in a defender with him. Before the pass had been made, Bergeron knew where the puck would be going. Hamilton scored through traffic, popping the lid on the Bruins’ comeback win.

“[Bergeron’s] got to make the decision — if I don’t see him coming through the middle and he’s covered, he’s going to go through and try to get open down low and try to give me an out in the corner,” Marchand said. “He knows that, and if he’s coming late, I might be able to hit him with a pass. It opens up a lot. It really creates some holes.

“It kind of confuses guys [on the other team] and guys have to make decisions. Bergy’s really good at reading off that.”

Marchand is a left-hand shot, which plays well off the righty Bergeron — the two pass back and forth easily, particularly on breakouts, where both keep their sticks ready to receive passes. They also have similar views of the game; it’s rare that both players will jump up in the offensive zone, leaving themselves open to a breakaway the other way. When one goes into the corner, the other is already sliding back.

It’s those four years, some 250-plus games of skating side-by-side, that lets them know how to play off one another.

“I think it’s everywhere,” Marchand said of how chemistry shows up. “It’s supporting one another in situations and battles. We kind of know where each other is. We don’t really have to look, you can just throw it blindly because you trust that he’s in a certain spot. On the defensive side of things, you know you’re covering up for each other and you know you can jump on the forecheck because you know a guy is covering up. It’s that trust factor that you know you take risks and guys will cover it up.”

Pieces fit

Milan Lucic had never scored more than 17 goals in his first three seasons in the NHL. In 2010, the 22-year-old was more known for what happened when his hands formed fists than when they gripped a stick.

Then the Bruins acquired Nathan Horton in the summer of 2010, and the idea was formed to put the twin giants of Lucic and Horton around David Krejci.

A boom was heard, and it wasn’t from Lucic planting an opponent into the boards. Lucic scored 30 goals in that 2010-11 season, turning into a scoring power forward that every team wishes they had. In three non-lockout seasons with Krejci feeding him the puck, Lucic has 80 goals.

Consider this — of Lucic’s 64 even-strength goals since 2010 (not counting empty-netters), 46 have been assisted by Krejci, or 71.9 percent. Lucic’s $6 million salary would be a pipe dream if not for his linemate.

“I think it’s made me a much better player,” Lucic said of his line. “You look at the talent and the skill that [Krejci] has, who knows if I would have the success that I've had if I wasn't playing with a guy like him? It's been a lot of fun and hopefully we can continue having great chemistry and great success moving forward.”

There was no guarantee that Lucic, Krejci and Horton would work. As Julien noted, there are plenty of different theories on putting together lines — a playmaker and a shooter, two playmakers and one shooter, a bruiser who creates room for two skilled players — but two power forwards with a playmaker was unusual; most teams just don’t have two skilled power forwards to even try it.

They clicked right away. In October 2010, their first month as a line, Lucic had four goals and four assists in eight games, Krejci two goals and seven assists.

“You kind of have to adjust with whoever it is,” Krejci said. “With Looch, it’s a fast game, up and down. He just drives the net hard, he's good on the forecheck. So I change my game a little — I dump the puck a little more for Looch, he's a big body. Once you've got 2-on-1s, 2-on-2s, 3-on-2s, you just drive the net or he drives the net, so he creates room for you.”

It would seem Krejci’s creativity might get stifled with two brutes around him. If it has, he’s found ways around it. With so much space to work with thanks to Lucic, Horton and Iginla, Krejci has the time to do his thing.

And just watch one game how often Krejci uses the drop pass to set up his wingers. Lucic has learned to stay in his lane, then burst behind Krejci as the center waits to send a pass back for a slap shot.

“I just know where he is most of the time,” Krejci said. “He's going into those spots where I am know where he's going to be, and I'm doing the same thing for him. Sometimes we throw each other blind passes, but it's not really that blind because one of us is there.”

The line has had its inconsistencies, particularly when Horton was on the right side. But Julien has never been tempted to pull the plug long-term; there’s too much invested in what’s already there, too much history of good times to think they won’t come back.

If Marchand and Bergeron have the ‘stop short’ play, then Lucic and Krejci have a set play off a faceoff. When Krejci takes an offensive-zone draw on the left side, Lucic is always behind the circle in a shooter’s crouch. Krejci wins the puck back to him, and while Krejci heads toward the net, Lucic lets it go.

It’s a play many teams run, but few as well as Lucic and Krejci. Krejci knows just where to put the puck off a draw, no easy task when the other center is chopping down on you.

“It's a play that we've ran for a long time,” Lucic said. “It's just one of those things that you don't even think about — you just sort of do it. That’s the trust thing, right? You trust that your linemate’s going to do the right thing and play the right way in the system. You don’t want a guy that’s cheating and not playing the right way, stuff like that. You just want to flow together in the right way so that things happen for you.”

This year, they have a new challenge. Iginla could not be re-signed, and Eriksson is their expected new linemate. Where Horton and Iginla wanted to smash-and-dash and play in straight lines, Eriksson is silky smooth, a pass-first player who glides around the ice.

There will be an adjustment, one both Lucic and Krejci will make, of course, together.

“With Looch, we click right away,” Krejci said. “With Loui, it's not the right word to say we have to find chemistry, but it's the right game that we have to find that suits us well with each other. That's what we've been trying to find.”

More to come

As he sat at his stall Saturday morning, undressing after the morning skate before their final preseason game, Krejci reflected on his lengthy stay with the Bruins.

He moved to North America from the Czech Republic in 2006 and was a full-time varsity player at 21 years old the next season. The 2007-08 season was also Lucic’s first in the NHL, that at 19 years old.

And Bergeron and Marchand are both closer to 30 than 20 now. The four most experienced forwards on the team are no longer the kids, going out for drinks after home games. Krejci, a mere 28, joked he feels old when all the new, younger players on the team don’t invite him and the members of his generation out anymore.

“Kind of changed a little,” Krejci said. “[Lucic has] got a 2-year-old daughter, so he’s a father-type of person, so he has changed a little. But he still finds time for his buddies, watching a football game at his place or just stuff like that. He’s just a great friend. He’s a tough guy on the ice, and he’s also a tough guy off the ice, but he’s got some soft side as well. He’s just a great guy, great friend.”

General manager Peter Chiarelli is no dummy. He knows who the good players are, and he knows which good players play well with other good players.

So Bergeron is signed through 2022, and Krejci’s new deal has him here through 2021. Marchand is in the second season of a four-year contract, and Lucic has another year left after this one before his deal is up.

And the Bruins will need all four to be on the top of their game this season. Bergeron and Marchand in particular had a strong training camp, with Chiarelli noting they were a step ahead of where they left off.

The bottom two lines are a ‘Who knows?’ group right now. Management is taking roster decisions right down to the wire, and the bottom six of Carl Soderberg, Matt Fraser, Chris Kelly, Daniel Paille and whoever else joins them won’t have much time to build chemistry before the puck drops Wednesday night and the treadmill of a hockey season starts rolling.

So Claude Julien will look to his Core Four Forwards to do a little more. And he’s got four years of evidence to suggest they can do that.

“It’s pretty hard to separate Marchand and Bergeron — and maybe someday it’ll happen — but it’s pretty hard to separate those two,” the coach muses. “You look at Lucic and Krejci, they’ve been a good pair over the years. Whether they have some bad games or not so successful at times in a season, still in people’s eyes it’s a pretty good line or a pretty good duo.

“I believe in that and that’s why sometimes it’s at least easier to replace one than to replace two.”