There are various ways in which calculators interpret keystrokes. One can categorize calculators into two main types: 1) single-step or immediate execution calculators and 2) expression or formula calculators. On a formula calculator one types in an expression and then presses 'Enter' to evaluate the expression. There are various systems for typing in an expression: infix, postfix, natural display, etc. On an immediate execution calculator, the user presses a key for each operation, by pressing keys to calculate all the intermediate results, before the final value is shown.