Infix notation: X + Y – Operators are written in-between their operands. This is the usual way we write expressions. An expression such as A * ( B + C ) / D Postfix notation (also known as “Reverse Polish notation”): X Y + Operators are written after their operands. The infix expression given above is equivalent to A B C + * D/ Prefix notation (also known as “Polish notation”): + X Y Operators are written before their operands. The expressions given above are equivalent to / * A + B C D