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Both terms are used to describe the arrangement of a structure with regard to the dimension of time. Historical developments, i.e., the succession or sequence of events over time, have diachronic [Greek dia + chronos, through + time] structure. The concurrent (here and now) existence and relatedness of elements, i.e., the contemporaneity of events, exhibit a synchronic [Greek syn, together] structure.

Definition extracted with permission from Simon, Fritz, et al, Family Process, Inc.: Language of Family Therapy: A Systemic Vocabulary and Source Book (Family Process Press Series)