The amount of business done between Premier League clubs quickly emerged as the defining theme of this summer’s transfer window. A staggering £1.6billion ($2.2bn) of spending stayed within the division’s ecosystem, including the British record-equalling £125million Manchester City paid Chelsea for Enzo Fernandez before Tuesday night’s signing deadline.It is an inevitable consequence of the competition’s growing financial dominance; few teams elsewhere in the world can afford the fees Premier League sides demand, forcing them to turn inwards. This has led to marriages of convenience and the thawing of relationships as clubs navigate this changed landscape. Aston Villa and Chelsea, for example, exchanged four players over the summer, with The Athletic reporting that the two organisations enjoy a strong relationship.Among the Premier League’s current 20 clubs, the Villa-Chelsea route is now the most well-trodden, with 13 transfers completed between the two while in the Premier League. Until recently, the direction of travel for these was largely from west London to Birmingham, through loans and modestly priced permanent transfers.But in an era when only six of the 20 teams spent under £100million this summer, those bargain-basement deals have given way to big-money moves. Both Chelsea and Villa broke their transfer record to sign a player from the other, with Morgan Rogers moving to Stamford Bridge and Nicolas Jackson heading in the opposite direction.Looking beyond that relationship to the league as a whole, the graphic below maps the number of deals between each of the current 20 clubs, with rows showing where players moved to and columns where they came from. Only transfers completed while both teams were simultaneously in the Premier League are included, with loans also counted.Again, Chelsea dominate.Many of the tiles involving them are shaded a deeper blue, denoting a higher number of deals. But it is their outgoings, represented by the column bearing their club badge, that stand out. Chelsea have sanctioned more moves to other Premier League sides than any other side, sending 10 players each to Everton and Villa.Arsenal have made nine signings from Chelsea, the most by one ‘Big Six’ club from another. The two London clubs’ transfer dealings got off to a controversial start with Ashley Cole’s infamous move from Arsenal, a deal clouded by accusations of tapping up and considerable hostility. A more cordial relationship exists now, particularly as Chelsea have relied on player sales to help sustain their lavish spending since owner BlueCo took over in 2022.Chelsea’s less selective approach to selling within the Premier League means Arsenal have sent £149million to their fellow Londoners, more than to any other domestic rivals. Manchester’s City and United have also spent more on Chelsea players than on those from any other Premier League club, at £178m and £184m respectively.Some heated on-pitch rivalries persist at the negotiating table, though.Tony Coton’s 1996 move to United remains the only transfer between the Manchester rivals in the Premier League era, while United and Liverpool have never exchanged a senior player since the competition began play in 1992. Neither have north London enemies Arsenal and Tottenham Hotspur.In monetary terms, that record-equalling Fernandez transfer means Chelsea and City now have the most lucrative relationship among Premier League clubs, with £303million spent between them. That surpasses the £244m exchanged between Chelsea and Manchester United, which had previously occupied top spot.In third place sits Brighton-Chelsea, but this transfer relationship has been heavily one-sided. Chelsea have forked out £236million on Brighton players, the largest sum any Premier League club has spent on signings from another, and all of it since 2022. That outlay has brought Marc Cucurella, Robert Sanchez, Moises Caicedo and Joao Pedro to Stamford Bridge, while their latest arrival from the south-coast side, centre-forward Danny Welbeck, was secured for a much more modest £5m this summer.