On the original calculators (HP35 et al.), the arithmetic keys (+ - x divide) are on the left side of the keypad. The keys are similarly located on my 41's. However, on my HP11 these keys are on the right hand side of the keypad, as they seem to be for all (?) later calculators.
For those of us (in the majority!) who are right-handed, it seems to me that having the arithmetic keys to the left of the numbers is most sensible. Did some left-handed HP engineer get into the calculator design about 1980 and change things around to his own chirality? Does anybody know why the change?