Abstract: That a pair of events in a system is said to be in concurrency is defined as "they can occur in any order" by R. Milner in CCS (Calculus of Communication System). This definition about concurrency is named as "interlacing concurrency". However, concurrency is defined as "disorder" by C. A. Petri in net theory. It is recognized as "true concurrency". In order to investigate the relationship and the difference between these two concepts, we used C/E system as models to discuss the appearance and essentiality of concurrency under both of these two definitions. As a result, a set of formulas to count the number of interlacing sequence (under the concept of interlacing concurrency) upon variable situations were given.

Key words: Concurrency, True concurrency, Interlacing concurrency, C/E system, Interlacing sequence