RPN stands for Reverse Polish Notation which was developed in 1920 by Jan Łukasiewicz (de.wikipedia) as a way to write a mathematical expression without using parentheses and brackets.

Hewlett-Packard (tm), realizing that Łukasiewicz's method was superior to standard algebraic(1) expressions when using calculators and computers, adapted RPN for its first hand-held scientific calculator, the HP35, in 1972.

- RPN saves time and keystrokes. You never have to account for the parentheses while doing calculations. The process is similar to the way you learned math on paper.
- You can see the intermediary results as you perform your computations rather than just the answer at the end. This is an extremely helpful byproduct. Math teachers are using this feature to improve student understanding of mathematics.
- An intermediate result allows the user to check the results and correct errors more easily. It's easier to follow the stream of calculation. The user defines the priority of operators.
- RPN is logical because the user first gives the number and then tells what to do with it.

RPN is also consistent in its usage. Most non-RPN scientific calculators are half RPN and half algebraic. For example, to perform addition, you need to enter 2+4 (algebraic), but to perform a sine calculation, you need first to enter the number and then to press the SIN button, which is a RPN method of entering the equation. Our RPN calculators do not suffer from this idiosyncrasy.

Here's an example:

__3+5__

7+6

Or (3+5) / (7+6) = x

Algebraic method: Add 3+5=8. Write down the answer or store it in memory. Add 7+6=13. Now enter the 8 from the first answer and then divide it by entering the second answer to get x=0.62.

RPN method: Press 3 then the ENTER key. Press 5 then the + key. Press 7, and then ENTER. Press 6 then the + key. Note that the answer to the second sum is displayed. Now here's the magic part. Press the divide key and the calculator gives the answer, 0.62.

Algebraic: 13 strokes, not counting the effort to write down or memorize the first answer while you calculated the second answer.

RPN: 9 strokes, and no need to write anything down.

Learning how to use an RPN calculator usually takes just a few minutes and it can save lots of time and effort over the long run.