<s>
Correlations	O
are	O
also	O
called	O
reciprocities	B-Algorithm
or	O
reciprocal	O
transformations	O
.	O
</s>
<s>
The	O
composition	B-Application
of	O
two	O
correlations	O
that	O
share	O
the	O
same	O
pencil	O
is	O
a	O
perspectivity	B-Application
.	O
</s>
<s>
If	O
a	O
correlation	O
φ	O
is	O
an	O
involution	B-Algorithm
(	O
that	O
is	O
,	O
two	O
applications	O
of	O
the	O
correlation	O
equals	O
the	O
identity	O
:	O
for	O
all	O
points	O
P	O
)	O
then	O
it	O
is	O
called	O
a	O
polarity	O
.	O
</s>
