Macros can have two types of operands: positional and keyword. A positional operand is written as a string of characters. This character string can be an expression, an implied or explicit address, or some special operand form allowed in a particular macro instruction. Positional operands must be included in a specific order. If a positional operand is omitted and another positional operand is written to the right of it, the comma that would normally have preceded the omitted operand must be included. This comma should be written only if followed by a positional operand; it need not be written if followed by a keyword operand or a blank.