In Spain, clubs can play their reserves in the same league providing they are in a lower division. Barcelona and Villarreal both have B teams playing in the second division and Real Madrid's second side, Castilla, are currently in the third tier.
Each club registers a squad of first-team players at the start of the season and a B-team squad. Once registered with the first team a player cannot then play for the second team. But a second-team player can be called up for first-team duty.
Pep Guardiola, the Barcelona coach, has given B winger Isaac Cuenca five first-team starts so far this season. But having been named in the first-team squad, another of the club's promising youngsters, the highly-rated Thiago Alcantara, cannot stay match-fit by playing for the second string.
The reserve teams cannot be promoted. If Barcelona's B team were to finish second this season, their promotion place would be taken by the side finishing third. The B team must also play in a division below the first team. This season Villarreal find themselves in the drop zone. If they end up going down, then Villarreal B will automatically be relegated, too
The reserve teams were previously also allowed to play in the Spanish Cup but in the 1979-80 season Real Madrid's B team reached the final where they met their senior counterparts. As Real won La Liga that year Castilla ended up in the European Cup Winners' Cup, playing West Ham. The Spanish federation has since banned B teams from the cup.