McLaren will field the most experienced driver lineup on the grid in 2016, with double world champion Fernando Alonso once again partnering 2009 champion Jenson Button.
The two veterans will roll onto the grid for the season-opening Australian Grand Prix with 537 Formula One starts, 47 wins, 147 podiums and 2,997 career points between them.
At a combined age of 70, they are by far the oldest pairing in the field—but mediocre racers do not last as long as they have. Only the Mercedes pairing of Lewis Hamilton and Nico Rosberg could claim to match the quality of the Alonso-Button partnership.
If McLaren and Honda can put together a good car, Alonso and Button are guaranteed to do great things with it.
But as this illustrious pairing prepares to once again do battle on the track in what should be one of the most interesting duels in the paddock, the future for both these giants of the sport is uncertain.
It's far from a given that we will see them together again in 2017—and even the winner of 2016's intrateam battle could be elsewhere when the cars line up at the start of next season.
Though there was always a chance 2015 would prove difficult for both drivers, it's unlikely either Alonso or Button suspected it would be quite as trying as it was.
McLaren's year got off to a bad start in pre-season testing, where they made headlines for all the wrong reasons. Their lack of running was bad enough—figures from the official F1 website reveal they managed just 1,751 kilometres across the 12 days—but they were also painfully slow.
The primary issue was their new Honda power unit, which was proving highly unreliable and seemed well down on power compared to its rivals.
But the McLaren chassis was not perfect either, and some kind of problem with the car—the nature of which was never made entirely clear—caused Alonso to crash heavily at Turn 3 of the Barcelona circuit on one of the few testing laps he managed to do.
The Spaniard sustained a concussion and was forced to miss the season-opening Australian Grand Prix. Rumours surrounding what had happened swirled, not helped by McLaren's handling of the situation. The team issued a statement blaming a gust of wind, and were forced to refute claims Alonso had lost control after suffering an electric shock.
However, Alonso himself told a different story. Speaking at the official pre-race press conference at the Malaysian Grand Prix, he blamed a problem with the car's handling and revealed changes had been made to the steering rack. He said:
Definitely we had a steering problem in the middle of turn three. It locked into the right and I approached the wall I braked in the last moment, I downshift from fifth to third, and yeah, unfortunately on the data we are still missing some parts.
Also the acquisition of date on that particular part of the car is not at the top so there are some new sensors here at this race and there are some changes we do on the steering rack and other parts and yeah that was the main thing.
Regardless of what happened, he returned and found himself driving a slow, unreliable car. Hopes that it would rapidly improve were dashed as the early races came and went, and it was soon clear there would be no significant progress until 2016 at the earliest.
As a result, the highly anticipated intrateam battle between Alonso and Button never had a chance to materialise.
They did, of course, compete with each other, but too often the power unit got in their way, ruining the chances of one or both men in either qualifying or the race.
Button took the early initiative in qualifying, beating Alonso in the first two sessions they both entered, but the Spaniard finished ahead of his team-mate first—coming home 12th to Button's 14th in China.
This sparked the start of a good Saturday run for Alonso, in which he outqualified Button six times in seven races. But the Brit was the first McLaren driver to get his name on the points table, grabbing four with an eighth-placed finish at Monaco.
Alonso scored his own first point of the season in changeable conditions at Silverstone, making the score 4-1 in Button's favour. But more telling than the points table was that, in the first eight races in which the two former world champions drove together, each had retired five times.
Furthermore, at this stage neither McLaren driver had made a single appearance in the final part of qualifying. The car simply did not have the pace to allow either man to compete for points on merit; though both Alonso and Button put in some sterling drives, their main weapon was luck.
They needed good fortune for themselves and poor fortune for the other runners—and at the 10th round of the season, the Hungarian Grand Prix, luck was very much on McLaren's side.
Qualifying was a disappointment; McLaren had expected to do well around the tight, twisty Hungaroring, but Button was knocked out in Q1 and lined up in 16th. Alonso fared little better, ending up 15th after his car broke down during Q2.
But the race was a different matter. While the rest of the field were busy crashing, breaking down or making silly mistakes, Alonso and Button kept their heads to bring their MP4-30s home in fifth and ninth, respectively.
No one was under any illusions that the result heralded the dawn of happier times, but McLaren had at last achieved a double-points finish.
Further opportunities presented themselves to both drivers in the remaining nine races as they fought out one of the closest—but most meaningless—intrateam battles on the grid.
Button grabbed ninth with a great run at the Russian Grand Prix, and he followed it up two weeks later with sixth in the dramatic United States Grand Prix.
The Austin race should have been McLaren's second double-score of the year, but Alonso's hopes were dashed, first by a collision with Felipe Massa at Turn 1, and later by the old McLaren favourite—a power-unit problem.
McLaren's season ended with a whimper, the team failing to score a single point in the final three races of the year. Given where they are in their careers, it was the worst campaign either driver had ever been forced to endure, but though their motivation suffered, their sense of humour did not.
At the end of the year, Button's four top-10 finishes left him on 16 points, but even this modest total was enough for him to come out on top in the intrateam battle. Alonso, who scored on just two occasions, only managed 11.
Some felt this was a fair reflection of the season as a whole—Keith Collantine of F1 Fanatic thought Button had been the better driver, rating the Brit one place ahead of Alonso in his end-of-year rankings.
Others held a differing view, believing Alonso had been superior. Autosport's Ben Anderson placed the Spaniard fifth in his seasonal rankings, with Button only 10th. The 10 team principals surveyed by the same publication ranked Alonso as the sixth-best driver of the year; Button didn't make their top 10.
B/R's own driver rankings, meanwhile, placed them neck-and-neck—equal eighth, with exactly the same race-average score.
Looking ahead to 2016, the McLaren driver lineup is unique in that it is the only pairing made up of two men who could both realistically choose to leave the sport of their own free will at the end of the year.
Furthermore, this is the only duel in which winning—either in terms of performance or in the points table—could be entirely irrelevant when it comes to deciding whether the two drivers will remain at the team in 2017.
Of the two, Alonso's future is the most secure; he heads into 2016 with the safety net of a solid, binding contract for 2017 in his pocket.
Per the Telegraph's Daniel Johnson, the Spaniard's deal with McLaren is worth around £25 million per year. Should he decide he wants to stay, he will—even in the unlikely event that he drives terribly in 2016, the team cannot afford to throw that much money down the drain to pay him to go away.
But there's at least a small chance that Alonso will decide for himself that he wants to leave. Toward the end of 2015, Motorsport.com's Jonathan Noble reported on speculation the Spaniard could leave F1 altogether; soon after, comments made by McLaren boss Ron Dennis to Sky Sports suggested the Spaniard could opt to take a sabbatical in 2016.
In the end, he stayed put, but what will he decide when the coming season draws to a close?
If the new MP4-31 proves competitive and the team's future looks bright, Alonso will want to be a part of it. But if the team have another year like 2015, and significant improvements look unlikely, he could call time on his 15-year F1 career.
And it's this decision, more than anything that happens on the track, that will shape McLaren's driver lineup for 2017.
Button's future in F1 has been the subject of speculation since the end of the 2014 season. Alonso was arriving and only one of the team's then-drivers—Button and young Dane Kevin Magnussen—would be kept on.
The Daily Mail's Jonathan McEvoy reported the team initially wanted to keep Magnussen to partner with the incoming Alonso.
However, McEvoy added that other factors, particularly those relating to sponsors and new engine partner Honda, swung things in the Briton's favour.
The speculation returned toward the end of 2015. Though Magnussen no longer looked to be in the picture, McLaren's other young star—newly crowned GP2 champion Stoffel Vandoorne—certainly was.
For a time, it looked like Button might take the decision out of McLaren's hands. When asked about his plans at the Singapore Grand Prix, he told Sky Sports F1:
It's a tricky one. When you ask the question it is always funny because I've been racing for so many years in F1 and you get emotional about the question because even if it is your decision or the team's decision to race in F1 next year you still get emotional because it has been your life for so long.
I always wanted to end my career on a high, but I also wanted it to be my decision when I retired. So it is a real tricky one, but at the moment this weekend I can't think about it anymore, I need to get my head down and race then I can think about it a bit more before Suzuka. Hopefully we can decide what the best thing for the future is for the team and myself.
After some thought, Button stayed—but regardless of how he and the team perform in 2016, the rumours and speculation will be in full flow once again by the time the summer comes around. Only this time, it seems more likely the decision will be made by the team.
Vandoorne is just too good a prospect to spend another year on the sidelines—holding him back until 2018 in favour of a driver coming toward the end of his career would be pure madness on the part of McLaren. There can be no doubt that the Belgian will be promoted to F1 in 2017, and if Honda haven't acquired a "B-team" by then—and it's unlikely they will have—he will be driving a McLaren.
That will almost certainly mean the end of Button's stay at the Woking-based team. Even if he wins the intrateam battle and wants to stay on, he doesn't have a secure contract for 2017; Alonso does, and it would cost McLaren too much to rip it up.
If Alonso stays and McLaren cannot place Vandoorne somewhere else, Button will go. He might opt to leave the sport altogether, or an opening could come up at another team.
But if Alonso decides to leave, or if Vandoorne can be placed somewhere else, it's likely McLaren will try to hold on to Button. He may be one of the oldest men on the grid, but he's still good enough to compete at the top level for at least a few more years—and the list of younger drivers who could come in and do a better job straight away is small.
However, whether Button will want to stick around in an uncompetitive car is another matter entirely. From what we saw of him toward the end of 2015, he probably wouldn't—and McLaren could end up losing both their star drivers.
Everyone knows how good Alonso and Button are, and neither is going to suddenly become a bad driver over the 21-race season ahead—but it's so difficult to see their partnership enduring for a third year.
If frustration doesn't break it up, Vandoorne surely will.
But whether it holds any meaning or not, this will still be one of the best fights we see all year. Both will want to do the best they can, and there'll be no holding back as they scrap to be the top dog at McLaren.
After all, for one or both, it could be the last intrateam battle they ever have.