As the group stage of the Vitality Blast pauses before the quarter-final race comes to its conclusion next week, we round up the tactical story of the tournament so far, with analysis of three teams in each group and how their strategies have evolved from previous seasons.

North Group

Nottinghamshire - spin to win at the new Trent Bridge

Nottinghamshire's signing of Imad Wasim has left them with a distinctly spin-heavy attack. In their victorious 2017 campaign, spinners bowled 36.5% of their overs; this year, that figure is up at 47.5%. With Matt Carter - a 6ft 4in offspinner - complementing Imad's darts and Samit Patel's flight, they have all bases covered, and Carter and Imad's regular powerplay overs have contributed to Notts being the second-most parsimonious team in the first six overs.

We are more than used to seeing run-fests at Trent Bridge, but after hosting five World Cup games, the pitch has been noticeably slower-scoring, with a scoring rate of 7.96, down from 8.66 last year and 9.86 in 2017.

The batting is yet to click, but Jake Libby has been substantially more important than his stats - five games, one innings of 24 off 28 balls - suggest. His presence in the side is similar to Billy Root's in the 2017 team: he is an insurance policy, constantly ready to be deployed in the event of a collapse, and by picking him Notts effectively give their fast-scorers license to attack more.

Durham - CLS: Chester-le-Street or Classically Low-Scoring?

Only Beckenham - which has only hosted two games - has hosted slower-scoring games than Chester-le-Street, where the average scoring rate is 6.89. That home advantage is yet to bring a noticeably strong home record - with two wins and two losses there - but has stood them in good stead for other low-scoring pitches.

The other keys have been their Australian contingent. Nathan Rimmington, the diminutive, 36-year-old seamer, has the second-best economy of death bowlers (7.07), and Peter Handscomb has contributed in the middle order, but D'Arcy Short has really held the key.

In the four matches Durham have won, Short has 261 runs at an average of 87.0 with a 163.2 strike rate and his left-arm wristspin goes at 7.14; in their five defeats, he has 107 runs at 21.4, scoring at 105.94, with a bowling economy rate of 8.43.

Lancashire - a 2018 repeat, with added Maxwell

Lancashire's spinners bowled just over half of the team's overs last year, and that figure is still high this year, at 47.4%. That puts them second only to Notts on that front, but Lancashire barely use them in the powerplay, instead turning the screw through the middle overs.

Liam Livingstone has not yet hit peak form, but is hitting a boundary every 4.35 balls in the powerplay, and Glenn Maxwell boasts a superhuman 172.89 strike-rate in the middle overs, typically the hardest time to score quickly.

The key for Lancashire will be replacing Maxwell and James Faulkner for Finals Day if they get there, with both expected to return to Australia for the start of the domestic season. Toby Lester is capable of filling Faulkner's shoes (he is less capable with the bat, but Faulkner has only faced 11 balls in the Blast anyway) but Maxwell's absence will be keenly felt.

South Group

Middlesex - five bowlers, and back Malan

Middlesex's decade of T20 decay was epitomised by a disastrous 2018 season, when they won just twice and used a staggering 16 bowlers over the course of the competition. Dawid Malan said on Sky during last week's washout against Essex that he realised that had to change, and this year has been a stark contrast: 97% of their overs have been bowled by the same five men.

That has meant an important clarity in roles that has sent them on an excellent run: Mujeeb Ur Rahman has bowled 22 powerplay overs, and Tom Helm has bowled 19; Nathan Sowter has been used almost exclusively in the middle overs, with Toby Roland-Jones and Steven Finn for support; and Helm in particular has led the charge at the death.

The key for the knockouts will be replacing Mujeeb, who has left to play in Afghanistan's tri-series against Bangladesh and Zimbabwe - Mohammad Hafeez is also a competent powerplay offspinner, but is perhaps less reliable.

While not always economical - they have conceded 8.47 runs an over during the course of the tournament - they are regular wicket-takers. Middlesex have taken 71 Blast wickets this season, nine more than second-best Somerset and more than double the number Essex (30) and Yorkshire (29) have managed.

With the bat, AB de Villiers and Eoin Morgan have been influential when available, but Malan has been crucial. He scores steadily in the powerplay, at a strike-rate of 106.73, and then flies into the middle overs, going at 177.58 and allowing only one dot ball per 5.5 deliveries faced.

Somerset - all change, but trust the three musketeers

Gradual evolution has been the story of Somerset's past few seasons in all forms, but in T20 there has been a big shift in personnel. From last season's regular XI, they are without Steven Davies, Peter Trego (both dropped), Johann Myburgh (retired), Corey Anderson (not re-signed) and the injured Lewis Gregory, while Jamie Overton has been left out recently.

Given Gregory and Anderson were crucial with the bat last year, they have had to tear up last year's gameplan of setting things up for a late blitz, and instead are trying to score the bulk of their runs early on, with Tom Banton and Babar Azam the competition's two leading scorers.

Spinners Max Waller and Roelof van der Merwe have been typically cheap, but their death bowling has been poor - their collective economy rate in the last five overs is 11.39, better only than Essex's, with Jerome Taylor leaking 159 runs in 67 balls at that phase.

Sussex - Mills brilliant, Briggs underwhelms

Danny Briggs became the leading all-time wicket-taker in English domestic T20 this season, but has had a poor Blast by his high standards - for only the second campaign in his career, his economy rate has been above 7.5.

On the flipside, Tymal Mills has been a revelation, not only in his usual death-bowling role but in all phases. His economy rates by stage are 4.60 (powerplay), 6.37 (middle), and 7.60 (death), and according to ESPNcricinfo's Smart Stats he is the competition's best seamer with 20+ overs to their name, with a Smart Economy Rate of 4.56.

Replacing him, therefore, is a tough ask, after the news he will be out for the season. The club is expected to announce that Jason Behrendorff will sign as a replacement overseas player, and while he is a powerplay specialist (econ. 6.80 since Jan 1, 2017), he has only bowled 20 balls at the death in that time period.

With the bat, Phil Salt has been typically quick out the blocks, with a strike-rate of 154.28 in his first five balls, and using Rashid Khan's death hitting has been a sign of improvement from last year. Rashid has been dismissed 5 times in the 17 balls he has faced at the death, but has also whacked 32 runs in that time; he is perhaps the ideal T20 tailender.