Toitsu 「対子」 are generic tile pairs. They may occur at any time during the development of a hand. Regardless, every hand requires at least one pair for completion, even kokushi musou.
Main article: Scoring
Specific tile pairs generate fu. Pairs of yakuhai tiles count for 2 fu.
Main article: Chiitoitsu
One yaku is specifically defined as a collection of pairs. Chiitoitsu is directly translated as "seven pairs".