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HP's scientific calculators were quirky in that most of them until quite recently solely or primarily used postfix or Reverse Polish Notation (RPN), a stack-based system in which every operator followed its operands: to add 2 + 3, instead of

2 + 3 =,

you'd press

2 [ENTER] 3 +.

These calculators had a stack, that is, a list of numbers represented as a vertical tower that they would generally enter and exit from the "bottom" (in HP's terminology, at least). In the addition above, the ENTER key would signal that you were finished typing a number and put the 2 on the stack. The + would implicitly do an ENTER if necessary, then take the bottom two items off the stack and add them.

For more complex calculations, there was no messing around with order of operations or nested parentheses--the stack was everything. So, say, (4+5) * 3 - 2 could be entered as

4 5 + 3 * 2 -

(where it's understood you type ENTER after every number not followed by an operator.)

The joke was that an advantage to using an HP was that nobody ever asked to borrow your calculator more than once. But for complex calculations off the cuff, people who used them soon learned to prefer RPN to algebraic entry, and I still like it.