Borussia Dortmund have agreed to pay Bayern Munich a reported €38million for Mats Hummels, but who else has played for both German giants?
Mats Hummels is set to return to Borussia Dortmund from Bayern Munich, joining former team-mate Mario Gotze back at Signal Iduna Park.
Hummels and Gotze were both part of the Dortmund teams that won back-to-back Bundesliga titles under Jurgen Klopp, so Lucien Favre will hope they can lead BVB back to the top.
Deals between the two German giants are not very common, but there have been some high-profile examples of players to feature for both Bayern and Dortmund.
Omnisport picks out six of the best, with Hummels' return costing Dortmund a reported €38million fee.
Hummels will know his way around Signal Iduna Park as it is only three years since the former BVB captain left Dortmund for Bayern. And he had already moved in the other direction earlier in his career. It is often forgotten Hummels started his career at Bayern only to sign for Dortmund, initially on loan, in January 2008. There, he would become one of the most impressive young defenders in the game, so it was no surprise Bayern bought him back in 2016.
Even at the age of 30, Dortmund will hope Hummels has more to give, especially as he will no longer be playing international football after Joachim Low's decision to drop him from the Germany squad. Hummels could make his next Dortmund debut against Bayern in August's DFB-Supercup.
Like Hummels, Lewandowski made his name at Dortmund and played a key part in their double title-winning success in the Klopp era. And, like Hummels, he was tempted away by Bavarian giants Bayern, making the switch in 2014.
Since then, the Poland international has developed a reputation as one of the world's finest strikers, often scoring against Dortmund, but the last couple of years have seen him linked with a move away. It is possible his star is on the wane too as the 22 Bundesliga goals last term were his lowest return since the 30-year-old's first season at Bayern.
There will be a familiar face for Hummels in the Dortmund dressing room as Gotze is also back at the club. The playmaker, who scored the goal that clinched World Cup glory for Germany in 2014, left Dortmund for Bayern in 2013 but never truly settled at the club and it made sense for BVB to bring him back after three mostly unhappy seasons.
"I can understand many fans could not accept my decision [to leave Dortmund]," he said. "I wouldn't reach it today either." Although a metabolic disorder has kept Gotze on the sidelines, he still scored seven Bundesliga goals last term and has won the club's fans back over.
Germany midfielder Frings had short spells for both Dortmund and Bayern, but he is best remembered for his two long periods as a Werder Bremen player.
Dortmund signed Frings from Werder during the 2002 World Cup but after two years he left for Bayern, where he won a domestic double. It was his only season in Munich, though, and he was soon back at Bremen.
Even though he was from Dortmund, it was at Bayern where Nerlinger started his career, winning a brace of Bundesliga titles as well as the UEFA Cup in 1995-96.
Nerlinger joined hometown club Dortmund in 1998 but injuries affected his ability to make an impact and the midfielder, capped six times by Germany, was forced into early retirement in December 2005.
Euro 1996 winner Helmer joined Dortmund from Arminia Bielefeld in 1986, but only won the DFB-Pokal at the club. Bayern were interested in buying the defender in 1992 but Dortmund were unwilling to sell him to their Bundesliga rivals.
Instead, Helmer moved to Ligue 1 side Lyon, but he then switched to Bayern in controversial circumstances only three months later. Helmer would become Bayern's captain and won three league titles before an inauspicious end to his career at Sunderland.