The P series generally means when the 9mm version was coming out. At that time in the '80's especially, the 9mm was the rage. Other brands were out there of course, as in the Browning Hi- Power, the Sigs, the Beretta, etc. Ruger came out with the P85, to want to compete with these and also to take part in the US Army tests for the new handgun service weapon. To me, the term "P" series term originated here. ......Others can correct me if I am wrong, but I think that is the way it came down ......I am not referring to the 22 semi auto that put Ruger on the map way back when. Sure Ruger had semi autos, but not in the "larger bore 9mm type".......And thank goodness, I am not on any meds.......Good luck with the P90.