It truly was a day for hundreds in English cricket as three Bob Willis Trophy centurions all did their bit at county level to emulate England's record-breaking duo Zak Crawley and Jos Buttler.
Glamorgan youngster Callum Taylor stole the show in the fourth round of this summer's new red-ball competition by hitting 106 on his first-class debut against Northamptonshire.
He shared a 124-run last-wicket stand with Michael Hogan, hitting 11 fours and six sixes in a 94-ball nightmare for Northants to rescue his side in the Central Group's battle of the bottom two.
Warwickshire skipper Will Rhodes was not far behind him, with a career-best 142 not out at Worcester as the Bears got on top of the Pears in their first red-ball meeting in five years, helping his side grind their way to 228-3 - at a run rate of less than two and a half per over.
But perhaps the most crucial of the three centuries might prove to be the 100 not out scored by Somerset's George Bartlett in one of the day's other derbies against Gloucestershire.
Bartlett's fourth first-class ton - and first at Taunton - rescued his side from 87-5 to help Somerset get to 237 all out. That still looked a disappointing home total until suddenly transforming into something comparatively huge by the end of the day when the visitors collapsed to 13-4.
Sadly, on this day of derby action, only 10 minutes of play was possible in the Roses Match.
Just 13 balls were possible between Yorkshire and Lancashire in Leeds.
They started at 11:30 BST, Tom Kohler-Cadmore made one scoring stroke - a full toss to the cover boundary off first-class debutant George Burrows - there were also four byes, and then off they came again at 11:40 with Yorkshire on 8-0 - and that was it for the day.
Much time needs to be made up on day two in this crunch meeting of second v third.
North Group leaders Derbyshire, who started the day just two points ahead of Yorkshire in the race to be the two sides who qualify for the final, were delayed a little by the weather but mostly by Durham opener Alex Lees at Emirates Riverside.
Ex-Yorkshire one-day captain Lees made 84 out of the North Group bottom side's 219-6 on a day shortened by the loss of the last six and a half overs.
In the East Midlands derby meeting of fourth v fifth at Leicester, visitors Nottinghamshire closed on 48-0 in reply to the 228 all out posted by Leicestershire.
The Foxes' total owed much to a late recovery from 54-5 thanks to half-centuries from Tom Taylor (57) and Ben Mike (51 not out).
Warwickshire had the better of the first day in the Midlands derby with group leaders Worcestershire - the first Pears-Bears fixture in first-class cricket since June 2015.
Rhodes' century was his first since he made 109 at Canterbury 14 months ago, when he shared a 221-run opening stand with Dom Sibley - the innings that propelled his Bears team-mate towards international selection.
But, although Ian Bell failed again, Rhodes did get good support from Sam Hain, who made a timely return to form with 55.
After the frustration second-placed Somerset had to endure at rain-lashed Edgbaston in midweek, Bartlett's superb innings at Taunton was a triumph for showing both patience and panache on a tricky batting surface against neighbours Gloucestershire.
But,in the first red-ball West Country derby in 13 years, he still needed help at the other end. And he got it from two of Somerset's battery of fast bowlers who all like to show they can bat a bit too.
After putting on 75 for the sixth wicket with Craig Overton, the real match-turner was the 61 Bartlett then shared with last man Jack Brooks.
Brooks was out the next ball after Bartlett had got to his ton - but that left the visitors having to bat out the final 12 overs and things did not go well, Overton and Josh Davey each taking two wickets.
In the other Central Group game, despite Wales-born, Australian-educated Taylor's stunning start to his Glamorgan career to help his team reach 259, hosts Northamptonshire then responded well on 82-1. Earlier, Northants pair Brett Hutton and Jack White both took four wickets.
Surrey left-armer Sam Curran returned from England Test duty at The Kia Oval to take 3-65 for the winless bottom club against third-placed Kent.
But, at a time of great joy for Kent batsmen, after not only Crawley's efforts for England but Jordan Cox and Jack Leaning's record-breaking stand a fortnight ago, it was their tailenders that had to rescue them.
Grant Stewart made 58 and Harry Podmore 47 from 42 balls in a 73-run seventh-wicket stand to help the visitors close on 295-8 after being put in.
A damp patch in the run-ups at the Castle End prevented any play in the top of the table meeting at Arundel between visiting temporary tenants Hampshire and South Group leaders Essex.
Hampshire are in second place, hoping to catch up with Essex, who are top after three straight wins, but have four other sides in the North and Central Groups within six points. Both teams were very keen to play but, despite four inspections, it was called off for the day.
On a day when runs came slowly almost everywhere, Sussex batted all day to be bowled out just before the close for 293 against Middlesex, of which Harry Finch top-scored with 69.