Numbers, enter, numbers, operation. That's about it, nice and simple. Programming in RPL was a lot of fun, sadly my HP-48G was lost to water damage (lost a bunch of things from that leaky roof). I still have my 32SII from college when a class required non-programmable calculators. Of course, it is programmable but they didn't know that (and I didn't use it) but it was easier to slip that out and stick with RPN than get used to a TI scientific calculator again.