This year: 4th in North Group; beat Gloucestershire in quarter-final
Last year: 6th in group stage
Most runs: Mark Stoneman (328)
Most wickets: Scott Borthwick and Paul Coughlin (17 each)
Highest total: 193 for 2 v Derbyshire
Highest individual score: Calum MacLeod (83)
Key features: One of three teams at Finals Day this year never to have won the tournament, Durham squeezed into the last eight thanks to a thumping win over Derbyshire and defeat for 2014 champions Warwickshire in the final round of the group stage. Durham have been scraping by all season - Paul Collingwood admits the club is "going through a difficult period" - but they are never better than when their backs are against the wall, as evidenced by winning the Championship in 2013 and Royal London Cup in 2014 despite ongoing financial trouble. Mark Wood's return to fitness after ankle surgery looks like a huge boost in the circumstances, the England pace bowler leading the way during Durham's quarter-final victory at Bristol, and Ben Stokes has also been made available as he recovers from a calf strain. Mark Stoneman, the former captain who will be playing his final limited-overs fixtures for the club before leaving for Surrey, looks to be coming into form at the right time, too.
Key player: While Stokes and Wood will provide the obvious X-Factor, Scott Borthwick might just be the man to unlock Finals Day for Durham. The legspinner has combined wicket-taking potency with a handy economy of 7.26 and, with Edgbaston having tended to aid slow bowling at this stage of the season, his craft ought to be to the fore when it comes to choking the scoring in the middle overs. Like Stoneman, Borthwick has attracted the attention of wealthier counties but he would surely love to scoop up some more silverware with Durham if he is to depart. Alan Gardner
The coach, Jon Lewis: "It's been a long time since we made it to Finals Day; too long, really. We've had decent teams in the past, but never quite done it. To reach it this year, with no overseas player, has been a really good achievement. I'd be lying if I said the money issues at the club has hadn't affected us: of course they have. People have been distracted. Players feel uncertainty and we all understand them being concerned over their future. Things were difficult for a few weeks. But the chief came and spoke to all the players and put some minds at ease. Mark Stoneman is leaving us, but I'm pretty confident we'll keep the rest of them. We have a proven record of producing players for England. There's no need to go anywhere else. Having Stokes back for Finals Day is huge - he'll play as a batsman only - and having Wood and Borthwick bowling in the middle of the innings - a proper fast bowler and good legspinner - gives us great wicket-taking potential. We used big boundaries at Durham - anywhere from 70m up to 90m - which plays to our athleticism. We'll never be satisfied just with being competitive. We want an expectation of winning."
This year: 2nd in North Group; beat Middlesex in quarter-final
Last year: beaten finalists
Most runs: Ben Duckett (389)
Most wickets: Richard Gleeson (14)
Highest total: 200 for 5 v Warwickshire
Highest individual score: Adam Rossington (85)
Key features: The team that keeps defying the odds to challenge in white-ball cricket. Making their third appearance at Finals Day in four years - they won in 2013 and made it to the final in 2015 - Northants' consistency comes despite the club's dire financial situation, the continuing interest of larger counties attempting to lure their talent elsewhere and, let's be honest, the less-than-svelte physique of several players. If Northants' results were reflected at a football club of similar size, it would be the manager - or in this case the coach - David Ripley gaining the offers. Ripley, who developed several of the squad at youth level, has somehow overcome the small playing staff (16) and lack of finance to build a selfless, confident team that achieved qualification to the knockout rounds of both of this season's limited-overs competitions. Their formula is built upon a commitment to aggressive cricket at the top of the order, with five men having contributed at least two half-centuries, several seamers who hit the pitch surprisingly hard and excellent fielding. But they may be without Richard Gleeson, arguably the revelation of the season as a fast bowler, who sustained a side strain on Thursday, and overseas legspinner Seekkuge Prasanna, who is required by Sri Lanka. Steven Crook is also a fitness doubt and they have not beaten their semi-final opponents, Nottinghamshire, for a decade.
Key player: As a business, Northants are an also-ran. But as a cricket club, they keep producing players. The latest to emerge from their production line is 21-year-old batsman Ben Duckett. Blessed with a hint of Jos Buttler and James Taylor, Duckett has been in prolific form this season, recently setting a new English record for the most List A runs (650) in a calendar month, during which he thrashed the highest one-day score by a Lions player, an unbeaten 220 against Sri Lanka. He is not out of contract this year, but is already attracting envious glances from richer counties wanting to entice him elsewhere. Finals Day provides a wonderful shop window to his extravagant talents. George Dobell
The coach, David Ripley: "We have played the underdog card quite well in recent years. I suppose that becomes more difficult with more sustained success: other teams know we are good these days and they know they can't take us lightly. We still make a positive out of a negative in terms of our small squad size: players aren't looking over their shoulders. They know they will be playing and what is required of them. That gives them confidence and the ability to learn their roles inside out. It is flattering that other clubs keep coming after our players; part of our role is to produce good players and we take great pride in that. But it's also frustrating. We've had a bit of a golden age of young players though the club - David Willey has shown his quality, Olly Stone and Ben Duckett will do the same - and it isn't easy to sustain that."
This year: 1st in North Group; beat Essex in quarter-final
Last year: 5th in group stage
Most runs: Riki Wessels (396)
Most wickets: Samit Patel and Harry Gurney (16 each)
Highest total: 185 for 8 v Lancashire
Highest individual score: Riki Wessels (80*)
Key features: This tournament represents the last chance to salvage success from a season that sees them facing relegation in the Championship. They overcame a quarter-final hoodoo by beating Essex when the game was appearing to slip away from them. However, they will need to hope a misfiring batting order has enough in them for two good performances on Finals Day. Providing he has overcome his frustrations of the Oval Test, Alex Hales' availability should help that side of things while Stuart Broad certainly has a 'big match' mentality and Andre Russell has won as many T20 tournaments as Jason Kenny has Olympic gold medals. In terms of squad, Nottinghamshire appear to have an embarrassment of riches with Imran Tahir and Dan Christian also available but the challenge will be to quickly refocus on the T20 format.
Key player: For all Russell's bling, Broad's match-winning bursts and Hales' ability to blast the ball, in Samit Patel Nottinghamshire have an experienced and extremely canny performer with bat and ball. Alongside Tahir he forms a telling spin combination; in the quarter-final he claimed four of the top six during Essex's collapse. Eoin Morgan has said that he wants extra spin options for the one-day squads over the winter, so Patel could yet force his way back into the England reckoning after his surprise recall to the Test side last year. Andrew McGlashan
The coach, Mick Newell: "People who say it must be great to have such an embarrassment of riches in selection terms aren't the ones who have to tell the players they will not be playing. We have three overseas players registered here but only two can play. With the weather poor and the possibility of shortened games, it may count against Imran Tahir. But we haven't decided yet. We haven't been good enough in the Championship of late - sides that are relegated tend to be sides that have batted poorly and we are batting poorly - but we have an opportunity to give our supporters something to celebrate and an opportunity to become one of very few sides to win all three competitions within a 10-year cycle. Winning trophies is tough but that is an exciting possibility."
This year: 3rd in North Group; beat Glamorgan in quarter-final
Last year: 8th in group stage
Most runs: Alex Lees (272)
Most wickets: Tim Bresnan (20)
Highest total: 223 for 6 v Durham
Highest individual score: Joe Root (92*)
Key features: Yorkshire were a mess in Twenty20 in early season, still looking for all the world like a Championship side uncomfortable in the shorter format. Then something clicked. Quite what is analysed in more depth elsewhere on these pages, but in essence a change of attitude also came with a change of personnel. David Willey, although a slow starter with the bat, has added a combative presence in the early overs with both bat and ball, Adam Lyth has been properly restored to the top of the order, Tim Bresnan can no longer be measured up so easily and the return of the offspinner Azeem Rafiq after a couple of difficult years out of the professional game has restored a partnership with Adil Rashid.
Key player: Even with Joe Root and Jonny Bairstow available for Finals Day (although Bairstow's hand took a battering in the Royal London Cup against Kent), it is hard to look beyond David Willey for Yorkshire's most influential player as he prepares for his fourth appearance at Finals Day. The barnstorming innings against Glamorgan in the quarter-final in Cardiff loudly stated that he wanted another and nobody was about to get in his way. Equally inspirational with the new ball and tigerish in the field, he fuelled Northants' challenge in the past and now he has done the same for Yorkshire. David Hopps
The coach, Jason Gillespie: "Why has our form improved? We gained greater clarity of our roles. We challenged everyone to think and understand exactly what they were trying to do and to stick to it once they got out there. It's tough to leave good players out, but everyone at Yorkshire understands that we have really good England players and, when they are available, they add to the side. It won't damage the spirit or the plans of the team. We've named the same 12 that played in the Royal London quarter-final. I think county cricket is a wonderful product and the quality of cricket is very high. The difference between it and the Big Bash is partly just the weather and partly about marketing. In Australia the Big Bash is on free-to-air TV. That has to make a difference."