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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Northants v Somerset 166
It's tea at Northants by the way, where Ben Sanderson led his side off the field after claiming 5-28.
Mightily impressive stuff from the paceman.
Image source, Getty Images
Worcestershire 175-3 v Glamorgan
Brett D'Oliveira reaches his half-century in a fourth-wicket stand that is worth more than a hundred with Jake Libby (81*) now.
Worcestershire in command at New Road.
Leicestershire 154-8 v Derbs
Dustin Melton runs in looking for a hat-trick.....
.....well blocked by Will Davis.
Still, a double-wicket maiden from Melton. He has a four-for.
Brooks c Rossington b Muzarabani 36 (Northants v Somerset 166)
Jack Brooks' average comes tumbling down...
He nicks Blessing Muzarabani behind after a huge swipe and that's the end of the Somerset first innings.
Brooks now averages a mere 54 in the Bob Willis Trophy but his last-wicket stand of 52 with Roelof van der Merwe could prove vital.
Northants v Somerset 160-9
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I mean, that's really silly, but factually correct.
Somerset's last man Jack Brooks is up to 34 and his average in this competition keeps on climbing.
Klein b Melton 0 (Leicestershire 154-8 v Derbs)
Woah Dustin Melton is on a hat-trick!
First he bowls Harry Swindells (21), then he cleans up Dieter Klein (0).
Clark c Podmore b O'Riordan 65 (Kent v Sussex 173-4)
Tom Clark's fine innings comes to an end.
The 19-year-old chips Kent spinner Marcus O'Riordan to mid-on and falls for 65.
It's a maiden first-class half-century for the youngster and it's put Sussex in a decent position at Canterbury.
Yorkshire 146-5 v Notts
We're also seeing some lovely shots too.
Harry Brook takes on Samit Patel and whacks him for six over long-on.
Brook moves to 57 and Yorkshire are 146-5.
It feels like the batsmen are starting to take a bit more control now. Like we said earlier before hundreds of wickets fell... a batting day.
Durham 111-5 v Lancs
Martin Emmerson
BBC Newcastle
For the second week in a row Alex Lees has prospered where his team mates have struggled.
It's the 44th time he's reached a first-class half-century. This one came in a patient 145 balls and included three fours. Last week he scored 106 against his former team Yorkshire.
Today he also put on a vital 52 for the fourth wicket with Gareth Harte, before Harte was run out on 18.
They say never run off a misfield but he did and paid the price as George Balderson recovered the ball from backward point to hit the stumps as he ran towards the keeper's end.
Northants v Somerset 153-9
Jack Brooks is some man to have coming in at number 11.
Fresh from his 72 against Glamorgan last week, Brooks has raced onto 29 off just 15 balls.
His last wicket stand with Roelof van der Merwe is worth 39 and this'll be frustrating Northants.
Yorkshire 138-5 v Notts
And now Harry Brook makes his half-century too.
He was part of a 92-run stand along with Jonathan Tattersall who's just fallen for 31 off the bowling of Samit Patel.
Yorkshire looked to be in serious trouble when Brook came to the crease at 33-3. However, his 88-ball 50 looks to have dragged his side back into this one.
Durham 106-4 v Lancashire
The 50s keep on coming. This time it's Alex Lees who hits a loosener from Liam Hurt for two to bring up his half-century.
He points his bat in the direction of the home dressing room.
A great knock so far from the 27-year-old.
Essex 136-4 v Surrey
Image source, Rex FeaturesAnd another maiden half-century for one of the county game's youngsters.
Essex's Feroze Khushi gets there in just his third first-class innings.
The 21-year-old is holding Essex's first innings together at Chelmsford.
Essex batsmen Feroze Khushi is closing in on a half-century. Essex are 133-4 against Surrey.
Sussex are in a good position at Kent - they're 155-3 led by Tom Clark's maiden fifty.
And at Radlett, Middlesex are 136-4 against Hampshire with Martin Andersson just bringing up his half-century.
Gloucestershire are faltering after lunch - they've fallen from 83-2 to 107-5 against Warwickshire.
As we've just mentioned, Somerset are in real strife at Northants - Tom Abell's men are 123-9 with Ben Sanderson picking up five wickets.
Worcestershire are in the driving seat against Glamorgan. Jake Libby (71*) and Brett D'Oliverira (40*) have taken the Pears to 151-3.
Harry Brook (40) and Jonathan Tattersall (30) are going along nicely for Yorkshire against Nottinghamshire. The pair are less than 20 runs away from a 100-run partnership. The score at Trent Bridge is now 127-4.
Durham are 102-4 against Lancashire with opener Alex Lees not out on 47. A few overs ago Gareth Harte was run out cheaply by George Balderson at point.
Leicestershire have slumped a little to 139-6 after being 84-3 against Derbyshire. Ben Mike is the latest wicket to fall at Grace Road, being bowled by Dustin Melton for 14.
Northants v Somerset 114-9
This Somerset first innings is folding quickly.
Josh Davey is the ninth man to go as he edges a Luke Procter delivery to slip.
Kent v Sussex 142-3
It's a maiden fifty for Sussex teenager Tom Clark in just his fourth first-class innings.
The 19-year-old left-hander gets there off 77 balls and has struck eight fours. Impressive stuff from the youngster.
Hankins lbw b Hannon-Dalby 11 (Gloucestershire 102-4 v Warks)
Two in two for Warwickshire as the first ball of Oliver Hannon-Dalby's over brings a wicket.
George Hankins is struck on the pad and falls for 11.
Gloucestershire were looking fairly comfortable five minutes ago.