Hand-out
The short version
A lost serve. In GAA rules a side-out means both doubles partners have lost it.
Related
Ref questions
A regulation game is played to:
Answer: 21 points
USHA games are to 21; the tiebreak is to 11.
The tiebreaker (third game) is played to:
Answer: 11 points
The deciding game is to 11.
A match is best of:
Answer: 3 games
Best two of three games.