A schedule S of n transactions is serialisable if it is equivalent to some serial schedule of the same ‘n’ transactions.
A serial schedule is a schedule in which either transaction T1 is completely done before T2 or transaction T2 is completely done before T1.
Any schedule that produces the same results as a serial schedule is called a serialisable schedule.
There are two possible serial schedule in case of two transactions (T1 and T2). That is T1-T2 or T2-T1. Similarly, in case of three parallel transactions the number of possible serial schedule is 3!, that is 6.
These serial schedules can be: