England
The champions are the favourites despite being without five leading forwards for part or all of the tournament, but the last time England won in Cardiff and Dublin in the same campaign was in 2003, when Eddie Jones was coaching that year's World Cup hosts, Australia.
There will be no hostile receptions for his current team in the two capitals, with supporters confined to their armchairs, but if that blunts an edge for the home sides, so Scotland and France will not view a trip to Twickenham with a feeling of helplessness despite not winning there in the Six Nations since 1983 and 2005, respectively.
England have not had the smoothest of buildups: none of their players has tasted action for at least a month, far more for most of their Saracens contingent, while Jones and his forwards coach, Matt Proudfoot, had to isolate last month.
England are enjoying their most profitable period since Italy joined the tournament in 2000, with three titles in five years and only out of the top two once in the last 10. Able to overcome injuries more seamlessly than any of their rivals, but their attack revolves around kicking and counterattacking and they remain unconvincing when playing catch-up and having to improvise.
Not as comfortable in possession as France or even Scotland, but Jones is counting down to 2023, when he will expect every facet of his side to be at its most piercing at the World Cup in France. One step at a time but there should be no limit to England's ambition.
France
Second to England last year in both the Six Nations and the Autumn Nations Cup, Les Bleus never resembled the indifferent bunch who meandered through the tournament in the years after the 2010 grand slam success.
France are reborn, organised, focused, prepared to express themselves and picking playmakers at 10 again rather than deploying scrum-halves or kickers in the position. They will be without Romain Ntamack but the player who will take the most replacing is the centre Virimi Vakatawa, a direct runner whose ability to storm the gainline provided a source of quick possession for Antoine Dupont to exploit.
The scrum-half was the player of the 2020 tournament and has rewired the position for France, who over the France have used their No 9s as the bandleader, rather than the 10s. Dupont can box-kick with the best of them but he thrives on chaos, ready to run at a defence that is not fully loaded.
France would probably have won the grand slam last year but for a red card at Murrayfield and it was indiscipline that cost them in the 2019 World Cup. That is their nature, moments of individualism either counting or costing. They are the closest to England of the other six and are playing without fear or inhibition.
Ireland
Last year seemed a hangover from the World Cup as the Irish tried to wriggle out of Joe Schmidt's tactical straitjacket and fit into something more comfortably tailored by Andy Farrell and Mike Catt.
They twice played Wales and Scotland in Dublin and won each time, but twice lost at Twickenham without securing a bonus point and ended the Six Nations with a 35-27 defeat in Paris, the highest number of points they had conceded in a championship match for 14 years.
That summed up their transition from a side that looked to keep the ball for several phases or test their opponents in the air. It was methodical, systematic and effective, but the breakdown is now more of a contest for possession and against England and France they lost the physical contest.
Farrell has drafted in the former Ireland and Lions captain Paul O'Connell to add steel to the forwards, but they will be without the No 8 Caelan Doris, a player expected to make an impact this year, for at least the opening match in Cardiff and that will hamper their desire to play with more pace.
Another issue for Farrell is at half-back, with Johnny Sexton and Conor Murray both trying to defy the passing of time. France and England travel to Dublin, but Ireland have to drive themselves with spectators absent. A test for Farrell now is as selector.
Italy
Franco Smith spoke impressively at the Six Nations launch and it is to be hoped the head coach's employers were listening. The Italian rugby federation has too often in the past thought that changing the person in charge of the team was all that was required, but attention is now being paid to the tiers below thanks to Conor O'Shea, Smith's predecessor.
Italy have become more of a force at under-20 level and while Treviso's improvement in recent years has stalled this season, the club has assembled a strong roster of coaches for 2021-22, including Paul Gustard.
For once, Italy look to have a plan. While it may not bring them a first victory in the Six Nations since 2015 - although at a time when the old normal has been suspended anything may happen - an essentially young side should not be weighed down by past failure.
Although there are some older hands in Luca Bigi and Carlo Canna, Smith is investing in the future. Jake Polledri will be missed but progress for Italy will be measured less in results and more in whether they can go the distance, having tended to blow up in the final quarter.
Scotland
The most intriguing of the six and, for once in a Lions year, Scotland should be worth more than a few token selections. Poor away form has cost them in the past but their success at a crowd-free Parc y Scarlets last October, their first Six Nations victory outside Murrayfield other than in Rome since 2010, means they should not be tiptoeing on to Twickenham this Saturday.
England have lost only once at home in the Six Nations since 2012 but in the autumn they tended to keep opponents at arm's length rather than put them away, winning through defence rather than attack. England will be the more rusty of the two sides and if Scotland rediscover the touch they showed in the second half at Twickenham in 2019, when they rallied from 31-0 to take the lead in a 38-38 draw, they will smoke out opponents who were not dragged out of their comfort zone during the Nations Cup.
Finn Russell is back, Cameron Redpath is an option in midfield and Stuart Hogg is playing with confidence, but Scotland are more than the sum of their backs, proficient now in the set pieces. The question for them is how they match up to the leading sides physically.
© Provided by The GuardianGregor Townsend during Scotland's pre-tournament training. Photograph: David Gibson/Rex/Shutterstock
Wales
Wayne Pivac's first Six Nations match in charge a year ago was a 42-0 victory over Italy, but their only successes since then have come against Georgia and Italy in the Nations Cup. Appointed as head coach with a remit to get Wales playing with flair, he will settle for results this championship, starting with Ireland in Cardiff on Sunday.
The New Zealander used the autumn to look at some emerging forwards, such as Shane Lewis-Hughes, James Botham and Sam Parry, but Dan Lydiate and Ken Owens have been recalled.
The future can wait but Wales's sense of adventure did not add up to much last year largely because their forwards did not deliver, especially in the lineout or at the breakdown. Wales were successful under Warren Gatland because they we able to impose themselves physically, in the Six Nations at least, and there is a sense of Pivac trying to recapture that while, in Johnny Williams and Louis Rees-Zammit behind, he has added zest.
Two questions: how much more can Pivac get out of the veterans who faithfully served Gatland? And can they find a way of generating the energy and ardour at the Principality Stadium that they failed to muster in Llanelli in the autumn? A team with such pedigree should be in the mix but, like Ireland, Wales are in a state of flux.
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