They are turns between speakers when they make a conversation.
for example Questions & answers. Greeting & greeting.
complaint & apology. e,g , first speaker says how are you ?(
this is pair) second speaker will answer, I am fine. according to
Sacks that he said some conditions about this topic. 1- They are
two utterances long. 2- The utterances are produced successively by
two different speakers. 3- The utterances are ordered. 4- The
utterances are related to each other.