<s>
Pitch	B-Application
circularity	I-Application
is	O
a	O
fixed	O
series	O
of	O
tones	O
that	O
are	O
perceived	O
to	O
ascend	O
or	O
descend	O
endlessly	O
in	O
pitch	O
.	O
</s>
<s>
It	O
's	O
an	O
example	O
of	O
an	O
auditory	B-Application
illusion	I-Application
.	O
</s>
<s>
However	O
pitch	O
also	O
varies	O
in	O
a	O
circular	O
fashion	O
,	O
known	O
as	O
pitch	O
class	O
:	O
as	O
one	O
plays	O
up	O
a	O
keyboard	O
in	O
semitone	O
steps	O
,	O
C	O
,	O
C	O
,	O
D	O
,	O
D	O
,	O
E	O
,	O
F	O
,	O
F	O
,	O
G	O
,	O
G	O
,	O
A	O
,	O
A	O
and	O
B	O
sound	O
in	O
succession	O
,	O
followed	O
by	O
C	O
again	O
,	O
but	O
one	O
octave	B-Language
higher	O
.	O
</s>
<s>
Because	O
the	O
octave	B-Language
is	O
the	O
most	O
consonant	O
interval	O
after	O
the	O
unison	B-Protocol
,	O
tones	O
that	O
stand	O
in	O
octave	B-Language
relation	O
,	O
and	O
are	O
so	O
of	O
the	O
same	O
pitch	O
class	O
,	O
have	O
a	O
certain	O
perceptual	O
equivalenceall	O
Cs	O
sound	O
more	O
alike	O
to	O
other	O
Cs	O
than	O
to	O
any	O
other	O
pitch	O
class	O
,	O
as	O
do	O
all	O
Ds	O
,	O
and	O
so	O
on	O
;	O
this	O
creates	O
the	O
auditory	O
equivalent	O
of	O
a	O
Barber	O
's	O
pole	O
,	O
where	O
all	O
tones	O
of	O
the	O
same	O
pitch	O
class	O
are	O
located	O
on	O
the	O
same	O
side	O
of	O
the	O
pole	O
,	O
but	O
at	O
different	O
heights	O
.	O
</s>
<s>
Roger	O
Shepard	O
achieved	O
this	O
ambiguity	O
of	O
height	O
by	O
creating	O
banks	O
of	O
complex	O
tones	O
,	O
with	O
each	O
tone	O
composed	O
only	O
of	O
components	O
that	O
stood	O
in	O
octave	B-Language
relationship	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
components	O
of	O
the	O
complex	O
tone	O
C	O
consisted	O
only	O
of	O
Cs	O
,	O
but	O
in	O
different	O
octaves	B-Language
,	O
and	O
the	O
components	O
of	O
the	O
complex	O
tone	O
F	O
consisted	O
only	O
of	O
Fs	O
,	O
but	O
in	O
different	O
octaves	B-Language
.	O
</s>
<s>
Circularity	O
effects	O
based	O
on	O
this	O
principle	O
have	O
been	O
produced	O
in	O
orchestral	O
music	O
and	O
electronic	O
music	O
,	O
by	O
having	O
multiple	O
instruments	O
playing	O
simultaneously	O
in	O
different	O
octaves	B-Language
.	O
</s>
<s>
showed	O
that	O
pitch	B-Application
circularity	I-Application
can	O
be	O
created	O
using	O
a	O
bank	O
of	O
single	O
tones	O
;	O
here	O
the	O
relative	O
amplitudes	O
of	O
the	O
odd	O
and	O
even	O
harmonics	O
of	O
each	O
tone	O
are	O
manipulated	O
so	O
as	O
to	O
create	O
ambiguities	O
of	O
height	O
.	O
</s>
