It introduced HP's third family of calculators after the original Classic family and the small Woodstock models, the Topcat family. It was not programmable, had 16 storage registers, a standard set of scientific and RPN functions, statistical functions, and of course the printing functions - print the X register, the stack, the statistics registers, and all registers. It had linear regression and H.MS arithmetic but no functions to convert between units. The printer was a thermal model - using temperature sensitive paper, more expensive than ordinary paper but saving the trouble of fiddling about to change a ribbon.

Since the HP-91 could not be called a "handheld calculator" the question arises whether it should be included in this series of articles about the history of handheld HP calculators - see the article "HP Calculator History - What Are We Talking About?". As befits a top cat, the HP-91 codename was "Felix".

This article is part of the WMJARTS file. This file contains a series of articles written by Wlodek Mier-Jedrzejowicz and published in DATAFILE, the journal of the HPCC. The article was reproduced with permission of the author.