Some background:  In 1971 Hewlett-Packard introduced the HP-35 hand held calculator.

It instantly replaced the slide rule, a mature device (invented in the early 1600s) 

universally used and revered.  It's been so long since a slide rule has been seen, 

that it's hard to remember that slide rules sent men to the moon.  My slide rule ($27 in 1952)

is essentially identical to my father's.  In spite of the $395 price, the HP-35 replaced

all slide rules in about a year, not because it was electronic and digital, but because 

it could do one thing a slide rule couldn't, that is, add.  At the $395 price, the HP-35 

tended to be a company capital equipment item.  In the mid seventies, HP came out 

with the HP21, small enough to fit in a pocket, and with a $125 price and I bought one 

for myself. I still have it; it works perfectly.  The HP21 program is a simulation of the 

HP21 calculator, except for the "help" button.

	The HP calculators use what is know as "reverse Polish notation" or RPN.  To add 
	
2+2, one hits 2,enter,2,then +, to get 4 i.e. the operation comes last.  At first, 

it would seem that this approach wouldn't be popular, but it turns out that, for long and 

complicated calculations, the RPN notation has a big advantage over the so called "algebraic"

or normal notation and all true nerds prefer the RPN method.

Texas instruments introduced a scientific "normal" calculator about the same time as the 

HP-35 although it cost only half as much as the HP, it was not popular among ex slide-rule 

pushers. However, as you know, in the broader market, it has become dominant.  In fact, I'm 

not sure one can buy a new RPN calculator now.
	   