A parallel variable is any data structure that contains a parallel array, for example, a parallel array of records, or an indexed array of parallel arrays. A sequential variable is any data structure that does not contain a parallel array. Similarly, a sequential statement is any statement that makes no reference to parallel variables. As mentioned in Section 3.3.2, parallel variables are distributed across multiple processors, and thus are the basic source of parallelism. Sequential variables, on the other hand, are replicated across the processors, but ZPL semantics guarantee that the multiple replicas remain coherent. By convention in this manual, all parallel variables have names that begin with an uppercase letter.