In REXX, a symbol that permits the substitution of variables within its name, when referred to. A compound symbol contains at least one period and at least two other characters. It cannot start with a digit or a period, and if there is only one period in the compound symbol, it cannot be the last character. The compound symbol begins with a stem (that part of the symbol up to and including the first period). The stem is followed by the tail (the parts of the name, delimited by periods, that are constant symbols, simple symbols, or null). Compound symbols allow the construction of arrays, associative tables, lists, and so on.