A rugby team is the sum of its individual parts. Between the forward and backline units, there are smaller groups again that must work collectively in the pursuit of victory.
But there are clearly key individual positions where a player must deliver individually; collectively, they make up the spine of a rugby starting XV.
The hooker, openside flanker, scrum-half, fly-half and fullback have the ability to influence a game considerably, in attack, defence and at, or from, set-piece.
We've ranked the spines from the top 10 teams that will turn out in this year's November Test series, while also adding a "floater" for each nation's most valuable player outside of those specific positions.
Looking further ahead, we've also factored in how things might change ahead of the Rugby World Cup next year, and made note of the players missing in action from this weekend through to the end of the Test window.
World ranking: 10
November spine: 2. Sam Matavesi 7. Peceli Yato 9. Frank Lomani 10. Ben Volavola 15. Setaraki Tuicuvu.
Spine ranking: 10
Floater: Leone Nakarawa. Brodie Retallick is peerless as world rugby's premier second-rower, but Nakarawa is certainly in the conversation as to the game's next best lock. The reigning European Player of the Year's 2017/18 season was so good that he was declared the No. 3 best player on the planet by Rugby World magazine.
Spine uncertainty: It may not happen this weekend in Edinburgh, but keep an eye out for playmaker Alivereti Veitokani to make an appearance at fly-half either off the bench or as a starter later on tour. Veitokani spearheaded Fijian Drua's run to Australia's National Rugby Championship title last month, topping the competition stats for metres run, try assists, offloads and clean breaks.
Missing: Not that they're short on talent in the outside backs, the absence of Nemani Nadolo this November will still be keenly felt. Now very much a senior player for Fiji, Nadolo has played all over the world and brings a huge amount of experience and an educated left boot while, better still, he is an exceptionally powerful ball-runner.
World ranking: 8
November spine: 2. Guilhem Guirado 7. Arthur Iturria 9. Baptiste Serin 10. Camille Lopez 15. Maxime Medard
Spine ranking: 9
Floater: Can Louis Picamoles get back to the form that once made him one of the best No. 8s in world rugby? If France are to get out of a tough Pool C next year he'll need to, but coach Jacques Brunel will be hoping the player's off-field behaviour is also on the improve; Picamoles was banished during the Six Nations after a night out in Edinburgh.
Spine uncertainty: It's France, right? Les Bleus made their name existing through prolonged periods of "uncertainty". At the moment, that certainly exists in the halves and at fullback, while skipper Guilhem Guirado is also under pressure from Camille Chat and Julien Marchand.
Missing: Long-time scrum-half Morgan Parra is a notable absence, at least for this weekend's clash with the Springboks, but it's Wesley Fofana whom Les Bleus could really use in their backline. Fofana is easily France's most dangerous "floater" attacking threat when fit.
World ranking: 9
November spine: 2. Agustin Creevy 7. Guido Petti 9. Tomas Cubelli 10. Nicolas Sanchez 15. Emiliano Boffelli
Spine ranking: 8
Floater: Having just been promoted to Pumas captain, free-running back-rower Pablo Matera looks set for a big run to next year's World Cup. He has been in sparkling form all year, firstly for the Jaguares and then the Pumas, and now gets the chance to really lead from the front.
Spine uncertainty: Just where does Argentina Rugby lie on its overseas policy? As it stands, the Union will allow European-based players to play Test rugby in "extreme cases" when they are not part of the Jaguares' Super Rugby squad. That change occurred midway through this year, but coach Mario Ledesma has included only Nicolas Sanchez, now of Stade Francais, of the Europe connection. Will it be the same next year?
Missing: While they're not short on quality back-rowers, Facundo Isa would certainly mount a series World Cup case. Having spent the past two seasons at Toulon, Isa is among those European-based players Ledesma is yet to call on in "extreme cases". Others include 2015 World Cup stars Santiago Cordero and Juan Imhoff.
World ranking: 7
November spine: 2. Fraser Brown 7. Jamie Ritchie 9. Greig Laidlaw 10. Finn Russell 15. Stuart Hogg
Spine ranking: 7.
Floater: He may be enduring one of his tougher weeks as a Test rugby player, but Huw Jones remains Scotland's key attacking weapon outside of Stuart Hogg. There are undoubtedly some defensive frailties, as seen last weekend against Wales, but he has the speed and footwork to cut the world's best defensive lines to pieces.
Spine uncertainty: Once a lock at No. 9 Greig Laidlaw is coming under increasing pressure from Glasgow's Ali Price at scrum-half. The good thing for coach Gregor Townsend is that the two halfbacks offer vastly different skill-sets; Laidlaw is an organiser who can kick goals, while Price offers a running threat around the fringes.
Missing: With next year's Six Nations earmarked as return date, John Barclay will be confined to the stands this this November. The Scotland skipper ruptured his Achilles tendon in May, ending his 2018 campaign.
World ranking: 3
November spine: 2. Ken Owens 7. Justin Tipuric 9. Gareth Davies 10. Gareth Anscombe 15. Leigh Halfpenny
Spine ranking: 6
Floater: The British & Irish Lions' Player of the Series in New Zealand last year, Jonathan Davies has legitimate claims to the tag of world's best outside centre. Only recently returned from a serious ankle injury, he marked his return to the Wales setup with a typical bustling break and try against Scotland last weekend. Another long-time backline threat is George North, whom Wales will hope can regain the form that saw him burst onto the scene as an 18-year-old.
Spine uncertainty: Kiwi-born Gareth Anscombe has earned the favour of his coach and now former countryman Warren Gatland in retaining the No. 10 jersey for this week's visit of Australia. But Dan Biggar shouldn't be written off, while Rhys Patchell have also proven he can step up and do a job when called upon. Fullback for the Lions in New Zealand last year, Liam Williams may yet displace Leigh Halfpenny in the No. 15 jumper, or shift to the wing himself.
Missing: The loss of the in-form Josh Navidi means Justin Tipuric has a clear run at the No. 7 jersey this November following former skipper Sam Warburton's retirement. Veteran No. 8 Toby Faletau is another player to fill the "floater" x-factor role; on his night the Bath back-rower is as good as any eighth-man in the game.
World ranking: 6
November spine:2. Tolu Latu 7. Michael Hooper 9. Will Genia 10. Bernard Foley 15. Dane Haylett-Petty
Spine ranking: 5
Floater: Israel Folau. The Wallabies superstar is never far from the spotlight, but it seems to have gone up another level in 2018. Still, he remains the Wallabies' key strike weapon and one whose versatility should not be overlooked, even if he is a "last resort" option as an outside centre. He'll start on the wing against Wales this week. While playing out of position at No. 8, David Pocock is still having a huge say on every Test he plays as his recent John Eales Medal can attest.
Spine uncertainty: There are two key areas of uncertainty for the Wallabies across the spine: hooker and openside flanker. Tatafu Polota-Nau remains Michael Cheika's go-to rake, but there are a growing number of options behind him as the Australian lineout continues to struggle. If former Under-20 captain Jordan Uelese can enjoy an extended Super Rugby run next year, he'll certainly come under consideration. The other key discussion point is whether Cheika can continue to pick skipper Michael Hooper ahead of David Pocock at No. 7; Isi Naisarani's eligibility clearance next April will only add to that.
Missing: Australia have been rather fortunate as far as injuries have gone in 2018, with no serious ailments sidelining anyone from their starting nucleus. Outside centre has been a poisoned chalice, however, with Reece Hodge following Samu Kerevi and Tevita Kuridrani onto the injured list. Kerevi has since returned, and he'll start against Wales.
World ranking: 4
November spine: 2. Dylan Hartley 7. Sam Underhill 9. Ben Youngs 10. Owen Farrell 15. Elliot Daly
Spine ranking: 4
Floater: When fit, there are few more damaging ball-runners than Billy Vunipola. The problem is, though, that he appears to be spending more and more time on the sidelines. For November, England's x-factor comes in the form of Manu Tuilagi, another player who has battled injury in recent years. Tuilagi has the ability to roll through the midfield, and generate quality front-foot ball for England.
Spine uncertainty: Just what does the future hold for Danny Cipriani? Many good English judges declare him to be the best playmaker in the Premiership this season, and he certainly provides a point of difference from current squad options Owen Farrell and George Ford. Will Eddie Jones turn to him for the Six Nations, or does Cipriani's checkered run off the paddock means his copybook is permanently stamped in the Australian's eyes? As for co-captain Dylan Hartley, he understands that, at age 32, there are no guarantees for next year's World Cup.
Missing: England were hit hard by injury ahead of the November Test window, with Anthony Watson joining the Vunipola brothers and Chris Robshaw as spectators. Watson would have certainly added to the mix at fullback alongside Daly and Mike Brown.
World ranking: 2
November spine: 2. Rory Best 7. Sean O'Brien 9. Kieran Marmion 10. Jonny Sexton 15. Jordan Lamour
Spine ranking: 3
Floater: The owner of the single-tournament record for Six Nations tries, Jacob Stockdale is among the game's premier finishers. Playing on the end of a backline that, at its peak, includes Conor Murray, Jonny Sexton, Gary Ringrose and Rob Kearney, Stockdale has proven he has the spatial awareness, speed and skill to run in five-pointers when the situation presents. Up front Tadhg Furlong continues to astound at tighthead prop.
Spine uncertainty: At the crux of Ireland's ascent up the world rankings under Joe Schmidt has been the settled nature of his squad, particularly across the key forward and backline positions. Looking ahead to the World Cup, once Conor Murray returns, there won't be too much change coming. Injury pending, of course.
Missing: Rob Kearney is missing this week as he still recovers from his shoulder injury, leaving Jordan Lamour to get another crack at fullback after his hat-trick against Italy in Chicago. He may yet return to face New Zealand, but the world's premier scrum-half, Conor Murray, won't be sighted this November. Just how badly Ireland will miss Murray, against the All Blacks in particular, will be interesting viewing.
World ranking: 5
November spine: 2. Malcolm Marx 7. Duane Vermeulen 9. Faf de Klerk 10. Handre Pollard 15. Willie le Roux
Spine ranking: 2
Floater: The Springboks at last evolved their game in 2018, leading to an upset win over the All Blacks in New Zealand, and that has brought their back three back into the game. And playing on the left wing has been Aphiwe Dyantyi, whose speed, footwork and finishing ability earned him a nomination for World Rugby's Breakthrough Player of the Year award.
Spine uncertainty: If all stay fit for the run to next year's World Cup, the Springboks will have the nuts and bolts of a fine starting XV in Japan next year. But like the Wallabies, coach Rassie Erasmus will need to find just what kind of blend he wants across his back-row. Does Pieter-Steph du Toit start at No.6 and Vermuelen at No. 8? Where does that leave Warren Whiteley? Meanwhile, as a good a player as Malcolm Marx is, his lineout throwing is increasingly becoming a concern.
Missing: The Springboks spine was certainly compromised at Twickenham last weekend given the Test with England fell outside the international window. But they have the European-based Faf de Klerk and Willie le Roux back at their disposal for this week's trip to Paris. Having beaten the All Blacks on the road this year, the crux of this Springboks line-up is a proven force.
World ranking: 1
November spine: 2. Codie Taylor 7. Ardie Savea 9. Aaron Smith 10. Beauden Barrett 15. Damian McKenzie
Spine ranking: 1
Floater: Scoring at better than a try per Test, Rieko Ioane is the most devastating attacking weapon in the game but so often the hard work is done inside him. For the All Blacks, much of that is driven by the peerless Brodie Retallick, who has reminded all of his ability this season after a difficult finish to 2017. Nominated for Try of the Year at the looming World Rugby Awards, Retallick's effort against the Wallabies was astonishing. At his best, he is a tackle-shredding, hard-hitting, turnover-winning machine.
Spine uncertainty: Part of the All Blacks' unrivalled depth is the fact that there will always be some discussion around their best starting XV. It's no different this November. At hooker, Dane Coles' return means Codie Taylor is suddenly on notice while Ardie Savea has the real chance to strut his stuff in Sam Cane's absence. There is also discussion around fly-half and fullback, but, unless something dramatic changes, you can expect Beauden Barrett and Ben Smith to fill those positions respectively when things get real in Japan next year, even if the latter spends more time on the wing over the next few weeks.
Missing: Cane is the only real key piece of the All Blacks puzzle sidelined this November, but Savea and fellow openside Matt Todd are more than handy replacements.