The first of the 20 series was essentially an HP-35 in a smaller, less expensive package. Since the HP-21 had 33% more ROM than the HP-35 it had room for a few extra features, namely:

- Trig functions in radian mode. (The mode was selected by a slide switch.)
- Display formatting. (DSP # displayed # decimal places in scientific mode. DSP . # displayed # decimal places in fixed mode.)
- Polar/rectangular conversions.
- Storage arithmetic. This was implemented using special keys for M+, M-, Mx and M/.
- 10
^{x}.

The HP-21 also implemented other changes that appeared in post HP-35 calculators. For example, it provided y^{x} rather than x^{y} and the CLR key left the storage register intact.

Front view (~47K)

Three quarter view (~79K)

Dimensions and Weight

Price: $125

Introduction-Discontinuation: 1975-1978

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