1
general::
Also, in turns. In the proper order or in sequence; also, one at a time. For example, Each generation in turn must deal with the same budget problems, or Someone must be awake at all times, so let's sleep in turns. [Late 1500s] Also see OUT OF TURN; TAKE TURNS.
American Heritage Idioms
2
general::
Phrase(s): in turn
in the appropriate point in the series or order; when one’s turn comes. • Someone has to wash the dishes after every meal. All of us will have to do it in turn. • All three of them shared the task of carrying water in turn.
McGrawhill's American Idioms And Phrasal Verbs