Local foes very rarely choose to do business with each other but in recent years Arsenal and Chelsea have flouted the unwritten rivalry rules, with several players having surprisingly moved between the two sides.
As Willian swaps Chelsea for Arsenal, we assess the impact of those who crossed between north and west London.
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The left back was the first player to move directly between Arsenal and Chelsea in the Premier League era in a move and transfer saga that sent shockwaves reverberating across the capital.
Amid tapping-up investigations and the publication of a verbal assault on Arsenal in Cole's autobiography, the defender moved to Stamford Bridge in acrimonious circumstances in 2006 for a measly initial £5million fee.
Cole remains vilified among Arsenal supporters to this day but his move to Chelsea was vindicated by winning one Premier League title, the Champions League, the Europa League, four FA Cups and a League Cup in an eight-year period in which the Gunners won just a single trophy.
Looking back on his controversial decision to join Chelsea, though, Cole said he had no regrets when speaking on Monday Night Football in February 2020: "I was a little bit hot-headed and didn't realise what I was doing at the time as well, but I don't regret it. When I look back now, it was something maybe that was meant to happen."
The Frenchman made an acrimonious switch to Arsenal as part of the deal which saw Cole move in the opposite direction, with Chelsea even claiming that the defender had refused to play for the club, before threatening to score an own goal if selected, in order to push through his transfer to the Emirates.
Gallas had played an integral role in Chelsea's first two Premier League title-winning campaigns and that experience made him a natural successor as captain when Thierry Henry left for Barcelona in 2007.
In his first season as captain, Gallas guided the Gunners to arguably their best Premier League title challenge since they were last crowned champions in 2004, but they missed out to Manchester United.
However, that was the closest Gallas came to silverware and he would go on to be stripped of the captaincy after a dressing room bust-up which eventually led to him leaving for Tottenham in 2010.
The France international moved to the Emirates on Deadline Day in 2007 but, just as he had during the three previous seasons at Chelsea, he struggled to nail down a regular first-team spot.
After five months and 13 appearances in all competitions, Diarra moved to Portsmouth in the January transfer window and was part of the squad that won the 2008 FA Cup.
Like Diarra, Benayoun's search for regular first-team opportunities saw him trade Chelsea for Arsenal.
A loan deal for the duration of the 2011/12 season yielded 25 appearances and six goals for the Israel international before he returned to Stamford Bridge and won the Europa League in 2013.
Four Premier League titles, four FA Cups, three League Cups and Champions League and Europa League winners' medals for good measure, Cech secured Chelsea legendary status in 11 trophy-laden seasons at Stamford Bridge.
The emergence of Thibaut Courtois limited his first-team appearances at Stamford Bridge and so Cech opted to join Arsenal for £10m in 2015.
"Life doesn't always turn out the way you think it will," he said of his decision to move across the capital. "It became clear that my situation would not improve and - as I know I am not at the stage of my career where I want to be on the bench - I made my decision to move and look for new challenges."
As for the Gunners, the capture of a seasoned winner, coupled with victory in the Community Shield against Chelsea in his first Gunners appearance, added fuel to the north London club's silverware dreams.
But Cech retired after defeat to Chelsea in the 2019 Europa League final having won just a single FA Cup and took up his current role as a technical and performance advisor with the Blues.
The Frenchman pursued a route out of the Emirates in Arsene Wenger's final season at the club as he built up form ahead of the 2018 World Cup, as the Arsenal manager explained when sanctioning his exit: "It was difficult on the family side for him to leave London. It was difficult to turn down (Chelsea's offer) just because it is an opponent.
"Olivier wanted more playing time - he wants as well to go to the World Cup. That is why I opened the door."
With Premier League champions Chelsea short up front following Diego Costa's acrimonious return to Atletico Madrid, Giroud became only the second player to make the switch in that direction after Ashley Cole, and his arrival at Stamford Bridge was viewed as a temporary stopgap.
"Chelsea is one of the biggest teams in the Premier League - they have won maybe the most trophies in the last six or seven years. It's a massive club," said Giroud of his decision to join the Blues.
Three-and-a-half-years on, however, and with World Cup, FA Cup and Europa League winners' medals to his name, Giroud has been proved right.
Arsenal brought in centre-back David Luiz from Chelsea on a two-year deal for £8m in August 2019 as manager Unai Emery sought to add some much-needed experience to his defence.
The Brazil international had made 248 appearances for the Blues across two spells with the club, during which time he won two Europa Leagues, the Champions League and the Premier League, while also becoming the only player in the club's history to have played in three European trophy final victories.
However, despite receiving criticism for some of his displays in his debut campaign at the Emirates, Luiz still finished the season by winning his first FA Cup after helping his new side beat his former employers 2-1 at Wembley in August 2020.
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As well as though seven players above, there have also been others who have featured for both clubs, but who did not move directly between the two teams:
While he didn't move directly between the two clubs, Cesc Fabregas' transfer to Chelsea from Barcelona in 2014 was viewed with disdain after purportedly coming at the expense of a return to Arsenal.
Having mastered his craft during 11 seasons in north London, Fabregas returned to boyhood club Barcelona in 2011, where he spent three relatively underwhelming seasons playing out of position and as understudy to Andres Iniesta and Xavi.
A return to the Premier League beckoned as the Spaniard looked to reignite his career and the Gunners had first refusal on midfielder, but it was an option they opted against taking as Chelsea swooped.
"I didn't want to be a beggar," Fabregas said of Arsenal's decision not to re-sign him. "They [Arsenal] know the situation, if in one week they hadn't said anything… straightaway I knew that probably they didn't want me back."
To be honest, thank God I took that decision. I know it can be controversial with the Arsenal fans, but these five years I've been very happy there
However, the move across London proved fruitless, with Fabregas winning two Premier League titles, an FA Cup and a League Cup during five seasons at Stamford Bridge.
"To be honest, thank God I took that decision," he said of his move to the Blues. "I know it can be controversial with the Arsenal fans but these five years I've been very happy there.
"We won everything in England. The most winning team in English football in the time I've been there, playing some great football at times."
Played for Arsenal from 1997-99 and joined Chelsea in 2008 after spells at Real Madrid, PSG, Liverpool (loan), Man City, Fenerbahce and Bolton Wanderers.
Enjoyed a hugely successful three-year career at Arsenal, before moving to Barcelona in 2000, only to then join the Blues just a year later.
The midfielder featured for the Gunners between 1985 and 1992 and after playing for Leeds United and Man City, he signed for Chelsea in 1994, where he spent four years.
Started his senior career at Arsenal in 2001, but after failing to break into their first team, the midfielder joined Reading two years later, before then signing for Jose Mourinho's Chelsea in 2007.
Summer transfer window - key dates and times
The summer transfer window will run for 10 weeks from July 27 and close at 11pm on October 5.
A domestic-only window for trades between the Premier League and EFL then runs from October 5 and closes on Friday October 16 at 5pm. Follow all the news and analysis on Sky Sports News and across Sky Sports' digital platforms, including with our dedicated Transfer Centre blog.