The two Athletics were so close during their early years that they were not allowed meet in official competition. They even shared players at times; the Madrid side's star forward Manuel Garnica Serrano was loaned to the Basques for the 1911 Copa del Rey final and he clinched the goal against Espanyol of Barcelona.
As Spanish football gradually became more formalised and professional, the two Athletics drifted apart. In 1921 the Bilbao side were crowned national champions after a 4-1 win over their Madrid brothers at San Mames. Full autonomy was well established by the time they were both became founder members of La Liga in 1928-29. A first top-flight meeting came on April 7, 1929 at the Estadio Metropolitano in Madrid, with the visitors winning 3-2.
Games between the two have often been fiercely contested through the decades, as most sibling rivalries can turn heated at times. A famous 6-6 La Liga draw in January 1950 saw Moroccan-born Larbi Benbarek among the scorers in a last gasp Atletico comeback.
Athletic's 4-1 victory at San Mames with 10 men in October 1982 was a key victory on the way to winning that season's La Liga title. A 2-1 victory at San Mames in April 2014 has been acknowledged by current Atletico coach Diego Simeone as the moment he knew his team really had what it took to take home that season's La Liga crown.
There have also been famous meetings in other competitions. The 1956 Copa del Rey final saw Athletic win 2-1 in Madrid. Atletico came out on top 3-0 when the two rojiblanco teams met in the 2011-12 Europa League final in Bucharest, their only meeting to date in UEFA competition.
Connections between the teams continue to this day. Former Athletic's sporting director Jose Maria Amorrortu was Academy Director at Atletico from 2006 to 2011, working with current first team stars such as Koke, before moving north and helping to develop local Basque talents including Inaki Williams and Iker Muniain.
Current Athletic player Raul Garcia made 216 La Liga appearances for Atletico between 2007 and 2015. The Pamplona-born midfielder's goal in the 1-1 draw at San Mames some seasons ago means he is one of very few to have scored for both sides in the fixture.
The sibling rivalry now continues on Saturday at Wanda Metropolitano, when Simeone's Atletico will face Marcelino's Athletic in La Liga. Fans and players on both sides are sure to enjoy the occasion, although past ties will count for little with three points on the line.