Four types of RPN

[Marcus von Cube]

The HP 35 has limited memory and uses its T register for intermediate calculations in some transcendental functions. It makes sense to clear T after its "abuse" in order to avoid presenting garbage to the user. So as long as you don't use any transcendental functions, the behavior is "top copy", but top can be cleared on occasion.

Edit: I just read in another thread that only the trig functions are affected and that the HP 35 duplicates Z to T after such an operation.