Athletic Bilbao B
Bilbao Athletic, officially named Athletic Club "B", is the reserve team of Athletic Bilbao, a football club based in Bilbao, in the autonomous community of Basque Country. The team will play in the Segunda División B in the 2018–19 season. Founded in 1964, the team holds home matches at the small stadium attached to the club's training facility at Lezama, or occasionally at San Mamés Stadium, with its 53,500-seat capacity, for important fixtures.
Atlético Madrid
The club was founded on 26 April 1903 as "Athletic Club Sucursal de Madrid" by three Basque students living in Madrid. These founders saw the new club as a youth branch of their childhood team, Athletic Bilbao who they had just seen win the 1903 Copa del Rey Final  in the city. In 1904, they were joined by dissident members of Real Madrid. The side began playing in blue and white, the then colours of Athletic Bilbao, but by 1911, both the Bilbao and Madrid teams were playing in their current colours of red and white stripes. Some believe the change came about because red and white striped tops were the cheapest to make, as the same combination was used to make bed mattresses and the unused cloth was easily converted into football shirts. This contributed to the creation of the nickname, "Los Colchoneros".
However, another explanation is that both Athletic Bilbao and Athletic Madrid used to buy Blackburn Rovers' blue and white kits in England. In late 1909, Juanito Elorduy, a former player and member of the board of Athletic Madrid, went to England to buy kits for both teams but failed to find Blackburn kits to purchase; he instead bought the red and white shirts of Southampton (the club from the port city which was his embarkation point back to Spain). Athletic Madrid adopted the red and white shirt, leading to them being known as "Los Rojiblancos", but opted to keep their existing blue shorts whereas the Bilbao team switched to new black shorts. Athletic Bilbao won the 1911 Copa del Rey Final using several 'borrowed' players from Athletic Madrid, including who scored one of their goals.