In tennis, the Alley is the area between the singles and doubles sidelines.
It is an extra strip of court space on each side. The alley is only used when playing doubles matches. In singles, balls landing in the alley are considered out.
Skilled doubles players like Bob Bryan and Mike Bryan use the alley strategically. They aim shots into the alley to make the court seem wider. The alley provides about 4.5 extra feet of court width on each side.