For two rivals in the same city, Arsenal and Chelsea do an awful lot of business together. 

When Kai Havertz moved from West London to the Gunners in the summer, he became the latest in a long line of Premier League stars to make the swap. 

Ahead of Havertz and his new team-mates' trip to Stamford Bridge on Saturday, we take a look back at the biggest transfers between the London giants. 

5. William Gallas (Chelsea to Arsenal, 2006)

William Gallas was part of the controversial transfer that saw Ashley Cole go to Stamford Bridge. 

It was a deal where acrimony was flowing in every direction, with Chelsea claiming that the French defender had threatened to score own goals if he was not allowed to leave. 

Gallas would go on to cause more controversy, leaving Arsenal to join Tottenham in 2010, to become the first footballer to play for all three of the London rivals. 

4. Olivier Giroud (Arsenal to Chelsea, 2018)

Olivier Giroud's sixth season at the Emirates saw the striker fall out of favour and sign for Chelsea midway through the 2017-18 campaign. 

Despite scoring 105 goals in 253 games for Arsenal and helping them to win three FA Cups, he was never a fans' favourite. 

The striker inflicted revenge when he scored for Chelsea in a 4-1 win against the Gunners in the 2019 Europa League final. 

3. David Luiz (Chelsea to Arsenal, 2019)

After two spells at Chelsea where he was equally capable of brilliance and calamity, David Luiz made an £8million move to Arsenal in 2019. 

In his first season, he was sent off against his former club after conceding a penalty, which was part of five spot-kicks that he gave away during 2019-20, breaking a Premier League record. 

The campaign would conclude with Luiz helping Arsenal to beat Chelsea 2-1 in the 2020 FA Cup final. 

2. Petr Cech (Chelsea to Arsenal, 2015)

After building a reputation as a Chelsea legend and winning a string of domestic and European honours, Petr Cech signed for Arsenal for a £10m fee. 

Some Blues fans were unhappy enough to issue death threats but that did not stop the goalkeeper winning the Golden Glove in his first season at the Emirates. 

As the game evolved, Cech struggled to play with the ball at his feet effectively. His last game before retirement was the 2019 Europa League final defeat to Chelsea. 

1. Ashley Cole (Arsenal to Chelsea, 2006)

The controversy over Cole's transfer started in 2005, when he was fined £100,000 by the Premier League after he and his agent met with Chelsea officials without permission from the Gunners. 

He would sign a one-year contract extension with Arsenal, before eventually moving to Stamford Bridge in 2006. The fee was set at £5m, with Gallas going the other way in part-exchange.

Having won two Premier League titles and three FA Cups in North London, Cole would add many more medals to his collection after his move west, including the 2012 Champions League.