# Number of operands to take when evaluating postfix notation?

• All sources I have seen list taking two operands from the stack once we encounter an operator.
• Why? What if not all of my operators are binary (taking two arguments)? What if I have custom unary and ternary operators?

I guess those sources innately assume binary operators. In a stack-based language (like postfix math notation) an $$n$$-ary operator takes $$n$$ items from the top of the stack as its input. Constants are often also viewed as arity 0, or nullary operators in this context.