<s>
The	O
HP-21	B-Device
was	O
a	O
scientific	O
calculator	O
produced	O
by	O
Hewlett-Packard	O
between	O
1975	O
and	O
1978	O
.	O
</s>
<s>
It	O
was	O
designed	O
as	O
a	O
replacement	O
for	O
the	O
HP-35	B-Device
,	O
and	O
was	O
one	O
of	O
a	O
set	O
of	O
three	O
calculators	O
,	O
the	O
others	O
being	O
the	O
HP-22	B-Device
and	O
HP-25	B-Device
,	O
which	O
were	O
similarly	O
built	O
but	O
aimed	O
at	O
different	O
markets	O
.	O
</s>
<s>
As	O
with	O
most	O
HP	O
calculators	O
then	O
and	O
now	O
,	O
the	O
HP-21	B-Device
used	O
RPN	O
entry	O
logic	O
,	O
with	O
a	O
four-level	O
stack	O
.	O
</s>
<s>
It	O
had	O
a	O
12-digit	O
LED	O
display	O
,	O
which	O
was	O
less	O
than	O
the	O
15	O
digits	O
of	O
the	O
HP-35	B-Device
.	O
</s>
<s>
Because	O
of	O
these	O
fewer	O
digits	O
the	O
HP-21	B-Device
(	O
and	O
similar	O
calculators	O
such	O
as	O
the	O
HP-25	B-Device
)	O
could	O
display	O
10-digit	O
floating	O
point	O
numbers	O
but	O
only	O
an	O
8-digit	O
mantissa	O
with	O
a	O
2-digit	O
exponent	O
when	O
scientific	O
notation	O
was	O
used	O
.	O
</s>
<s>
A	O
shift	B-Device
key	I-Device
provided	O
access	O
to	O
functions	O
whose	O
legends	O
were	O
printed	O
on	O
the	O
fronts	O
of	O
the	O
tall	O
trapezoidal	O
keys	O
.	O
</s>
