Commentary of every game available at top of this page
All 18 counties split into three regional groups of six
Each side will play five games
Two sides with most points play five-day final
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Middlesex 246-9 v Hants
Two wickets in two balls for Hampshire seamer Ryan Stevenson!
First Nathan Sowter cuts him to slip, before the wonderfully named Thilan Walallawita nicks his first ball behind.
Tim Murtagh sees off the hat-trick ball.
Yorkshire 264-9 v Notts
A fourth six for Jordan Thompson who pulls Jake Ball into the stands.
The youngster has 98 chasing his maiden first-class century.
Procter lbw b Davey 5 (Northants 65-6 v Somerset 166)
Wicket number 16 on day one!
It's another lbw as Josh Davey traps Luke Procter.
Surely Somerset won't be batting again before the day is out?!
Image source, Getty ImagesDerbyshire 11-1 v Leicestershire 199
Early breakthrough for the foxes as Derbyshire skipper Billy Godleman edges Chris Wright to Hassan Azad at first slip.
Godleman falls for nine. A perfect start with the ball for Leicestershire.
Yorkshire 252-9 v Notts
Yorkshire aren't going down without a fight.
It's consecutive sixes for Jordan Thompson who launches Samit Patel over wide long-on and then over mid-wicket and takes 20 from the over.
He moves to 86 from 95 deliveries.
Gloucestershire 160-7 v Warks
Another wicket for Warwickshire and another wicket for captain Will Rhodes.
Michael Burgess stumps Gareth Roderick who falls for 39.
Rhodes now has 3-15 from his 12 overs.
Just the bowlers remain for the hosts.
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Northants 53-5 v Somerset 166
Wickets are tumbling at Northants today!
Rob Keogh and Charlie Thurston both fall lbw in consecutive overs to Josh Davey and Jack Brooks and Northants are in strife.
53-5 and still trailing by 113.
Image source, Getty ImagesEssex 210-5 v Sussex
Paul Walter and Adam Wheater are building a solid platform for Essex.
It's slow going at Chelmsford but their sixth-wicket stand is now up to 45.
Surrey need a breakthrough from somewhere.
Yorkshire 232-9 v Notts
Image source, Rex FeaturesTwo quick wickets and Nottinghamshire are nearly done wrapping up Yorkshire's first innings.
Samit Patel traps Duanne Olivier lbw for five and then an over later Jack Shutt is dismissed for a duck the same way by Matthew Carter.
The visitors are now on 232-9.
Gloucestershire 160-6 v Warks
Warwickshire have their first wicket of the evening session as skipper Will Rhodes traps Jack Taylor lbw.
Taylor had made 14 from 74 balls at a strike rate of just 18.92.
Gareth Roderick remains at the crease for the hosts on 39.
Davis c Hosein b Reece 20 (Leics 199 all out v Derbys)
It was a great cameo but Will Davis eventually falls for 20.
And it means Leicestershire finish their first innings with 199.
Dustin Melton was the star amongst the bowlers taking 4-22.
Kent v Sussex 253-7
Image source, Rex FeaturesKent continue to chip away at Sussex.
Harry Podmore has just picked up his third wicket, snaffling Jack Carson at slip.
Sussex captain Ben Brown is the key... he's on 57.
B Curran c C Overton b Davey 35 (Northants 50-3 v Somerset 166)
That's the wicket Somerset wanted!
Ben Curran pokes at a delivery from Josh Davey and Craig Overton clings on low at third slip.
Somerset are fighting back nicely as the Northants opener goes for 35.
Worcs 222-3 v Glamorgan
HUNDRED KLAXON!
Worcestershire opener Jake Libby gets to three figures as his side continue to dominate against Glamorgan.
His stand with Brett D'Oliveira (80*) is worth 152.
Northants 46-2 v Somerset 166
Somerset needed that.
It's a mix up in the middle and Northants number three Ricardo Vasconcelos is run out thanks to a direct hit by Jack Brooks.
Ben Curran is looking good mind on 31.
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Andersson lbw b Fuller (Middlesex 201-7 v Hants)
Martin Andersson falls just short of a maiden first-class hundred.
The 23-year-old is pinned lbw by James Fuller for a career-best 92 and Middlesex are seven down.
Patterson c Mullaney b Nash 11 (Yorkshire 218-7 v Notts)
A second wicket for Chris Nash!
He hadn't bowled a ball in a first-class game for over two years before today. Now he's got two wickets.
Yorkshire captain Steven Patterson edges to Steven Mullaney and Notts are right back in this.
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Durham 141-7 v Lancs
The first over after tea and Lancashire already have a wicket.
George Balderson's away-swinger is edged by Durham captain Ned Eckersley to Keaton Jennings at first slip.
Eckersley goes for eight, and Lancashire are seemingly back in control.
Kent v Sussex 236-6
Image source, Rex FeaturesFifty for the Sussex skipper.
It's just about even-stevens at Canterbury, but a good hour for either side could turn the tide of this one.
Yorkshire 210-6 v Notts
Thompson slashes Zak Chappell between slip and gully to reach his first half-century off just 70 balls.
A great cameo from the all-rounder playing in just his third first-class fixture.
Yorkshire have also passed the 200 mark.