<s>
A	O
marker	B-Device
is	O
a	O
type	O
of	O
special	O
purpose	O
control	O
system	O
that	O
was	O
used	O
in	O
electromechanical	O
telephone	O
central	O
office	O
switches	O
.	O
</s>
<s>
Switches	O
employing	O
markers	B-Device
belong	O
to	O
a	O
class	O
of	O
switches	O
known	O
as	O
"	O
common	B-Device
control	I-Device
"	O
,	O
as	O
the	O
purpose	O
of	O
a	O
marker	B-Device
is	O
to	O
control	O
the	O
closure	O
of	O
contacts	O
in	O
the	O
switching	B-Architecture
fabric	I-Architecture
that	O
connect	O
a	O
circuit	O
between	O
the	O
calling	O
party	O
and	O
the	O
called	O
party	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
"	O
direct	O
control	O
"	O
switches	O
,	O
where	O
the	O
switching	O
elements	O
were	O
controlled	O
directly	O
by	O
the	O
customer	O
's	O
dial	O
,	O
such	O
as	O
the	O
Step	B-Device
by	I-Device
Step	I-Device
switch	I-Device
.	O
</s>
<s>
The	O
term	O
marker	B-Device
came	O
from	O
its	O
use	O
to	O
mark	O
a	O
path	O
of	O
links	O
through	O
the	O
switching	B-Architecture
fabric	I-Architecture
.	O
</s>
<s>
A	O
marker	B-Device
's	O
comprehensive	O
view	O
of	O
the	O
switching	B-Architecture
fabric	I-Architecture
allowed	O
it	O
to	O
find	O
and	O
assemble	O
a	O
path	O
from	O
one	O
terminal	O
to	O
another	O
,	O
if	O
the	O
links	O
were	O
available	O
,	O
unlike	O
the	O
earlier	O
graded	O
progressive	O
systems	O
in	O
which	O
a	O
path	O
might	O
not	O
be	O
found	O
.	O
</s>
<s>
Markers	B-Device
were	O
sometimes	O
referred	O
to	O
as	O
special	O
purpose	O
computers	O
,	O
but	O
lacking	O
stored	B-Device
program	I-Device
control	I-Device
,	O
they	O
were	O
not	O
computers	O
according	O
to	O
the	O
understanding	O
of	O
the	O
middle	O
20th	O
Century	O
.	O
</s>
<s>
1	O
Crossbar	O
in	O
1938	O
,	O
the	O
marker	B-Device
was	O
a	O
technological	O
descendant	O
of	O
the	O
decoder	O
that	O
was	O
used	O
previously	O
in	O
the	O
panel	O
system	O
.	O
</s>
<s>
Markers	B-Device
were	O
constructed	O
from	O
hundreds	O
or	O
thousands	O
of	O
relays	O
mounted	O
together	O
in	O
large	O
frames	O
.	O
</s>
<s>
Different	O
types	O
of	O
markers	B-Device
performed	O
various	O
specialized	O
hard-wired	O
operations	O
.	O
</s>
<s>
1	O
Crossbar	O
switches	O
had	O
separate	O
markers	B-Device
for	O
originating	O
and	O
terminating	O
calls	O
,	O
named	O
"	O
Originating	O
Marker	B-Device
"	O
,	O
and	O
"	O
Terminating	O
Marker	B-Device
"	O
,	O
respectively	O
.	O
</s>
<s>
5	O
Crossbar	O
switches	O
had	O
dial	O
tone	O
markers	B-Device
to	O
select	O
one	O
of	O
a	O
number	O
of	O
shared	O
digit	O
receivers	O
(	O
termed	O
originating	O
registers	O
)	O
and	O
connect	O
it	O
to	O
a	O
subscriber	O
who	O
wished	O
to	O
make	O
a	O
telephone	O
call	O
.	O
</s>
<s>
The	O
digit	O
receiver	O
collected	O
the	O
digits	O
as	O
the	O
customer	O
dialed	O
,	O
and	O
made	O
them	O
available	O
to	O
the	O
Completing	O
Marker	B-Device
,	O
which	O
used	O
them	O
for	O
routing	O
purposes	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
Completing	O
Marker	B-Device
would	O
mark	O
a	O
proper	O
path	O
of	O
idle	O
links	O
for	O
the	O
call	O
to	O
make	O
through	O
the	O
mechanical	O
voice	O
switching	O
matrix	O
.	O
</s>
<s>
Telephone	O
switches	O
that	O
employed	O
markers	B-Device
almost	O
always	O
had	O
several	O
of	O
each	O
type	O
required	O
by	O
the	O
machine	O
.	O
</s>
<s>
During	O
normal	O
operation	O
,	O
traffic	O
load	O
was	O
balanced	O
evenly	O
among	O
the	O
markers	B-Device
.	O
</s>
<s>
If	O
one	O
or	O
more	O
markers	B-Device
became	O
unavailable	O
due	O
to	O
a	O
trouble	O
condition	O
,	O
the	O
load	O
was	O
directed	O
to	O
the	O
remaining	O
markers	B-Device
for	O
handling	O
.	O
</s>
<s>
If	O
momentary	O
or	O
intermittent	O
trouble	O
was	O
encountered	O
,	O
a	O
marker	B-Device
usually	O
returned	O
an	O
error	O
condition	O
that	O
instructed	O
the	O
equipment	O
to	O
try	O
again	O
using	O
a	O
different	O
marker	B-Device
.	O
</s>
<s>
Markers	B-Device
were	O
used	O
in	O
the	O
design	O
of	O
switches	O
from	O
the	O
1930s	O
until	O
the	O
late	O
1960s	O
when	O
they	O
were	O
replaced	O
with	O
software	O
controlled	O
electronic	O
computers	O
of	O
modern	O
design	O
.	O
</s>
<s>
During	O
the	O
middle	O
20th	O
Century	O
,	O
markers	B-Device
in	O
Bell	O
System	O
exchanges	O
acquired	O
other	O
functions	O
that	O
were	O
only	O
needed	O
once	O
or	O
twice	O
per	O
telephone	O
call	O
,	O
including	O
outgoing	O
digit	O
translation	O
and	O
enforcement	O
of	O
different	O
policies	O
upon	O
different	O
classes	B-Device
of	I-Device
service	I-Device
in	O
the	O
provision	O
of	O
features	O
to	O
customers	O
.	O
</s>
<s>
This	O
practice	O
evolved	O
into	O
Customer	O
Groups	O
,	O
allowing	O
the	O
addition	O
of	O
Centrex	B-Device
features	O
to	O
the	O
No	O
.	O
</s>
<s>
These	O
were	O
the	O
most	O
complex	O
markers	B-Device
made	O
,	O
and	O
were	O
abandoned	O
in	O
the	O
1970s	O
and	O
1980s	O
when	O
Stored	B-Device
Program	I-Device
Control	I-Device
supplanted	O
the	O
marker	B-Device
as	O
the	O
primary	O
method	O
of	O
controlling	O
telephone	O
switches	O
.	O
</s>
<s>
Markers	B-Device
were	O
mostly	O
associated	O
with	O
crossbar	O
switches	O
,	O
but	O
many	O
non	O
Bell	O
System	O
crossbar	O
exchanges	O
did	O
not	O
use	O
them	O
.	O
</s>
<s>
Where	O
those	O
exchanges	O
had	O
markers	B-Device
,	O
for	O
example	O
in	O
the	O
British	O
TXK	B-Device
or	O
the	O
Ericsson	O
ARM	O
,	O
they	O
were	O
simpler	O
in	O
design	O
and	O
function	O
.	O
</s>
<s>
The	O
digit	O
translation	O
jobs	O
that	O
were	O
added	O
to	O
Bell	O
System	O
markers	B-Device
were	O
handled	O
by	O
other	O
equipment	O
.	O
</s>
