The design hearkens back to HP's classic calculators for which it is still well known, such as the HP-41C, HP-65 and 67, and to a lesser degree, the HP-35. Unlike the original HP-35, the 35s is programmable and has lots of memory. It's more like an HP-67 or HP-41CV in many ways, which has spawned some unfair (to my mind) criticism.
I like the HP-35s a lot, you can find out why, and what I think could stand improvement here.
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Webomator.com: Backward into the