<s>
A	O
66	B-Architecture
block	I-Architecture
is	O
a	O
type	O
of	O
punchdown	B-Protocol
block	I-Protocol
used	O
to	O
connect	O
sets	O
of	O
wires	O
in	O
a	O
telephone	O
system	O
.	O
</s>
<s>
66	B-Architecture
blocks	I-Architecture
are	O
designed	O
to	O
terminate	O
20	O
through	O
26	O
AWG	O
insulated	O
solid	O
copper	O
wire	O
or	O
18	O
&	O
19	O
gauge	O
skinned	O
solid	O
copper	O
wire	O
.	O
</s>
<s>
The	O
66	O
series	O
connecting	O
block	O
,	O
introduced	O
in	O
the	O
Bell	O
System	O
in	O
1962	O
,	O
was	O
the	O
first	O
terminating	O
device	O
with	O
insulation	B-Protocol
displacement	I-Protocol
connector	I-Protocol
technology	O
.	O
</s>
<s>
The	O
term	O
66	B-Architecture
block	I-Architecture
reflects	O
its	O
Western	O
Electric	O
model	O
number	O
.	O
</s>
<s>
The	O
25-pair	O
standard	O
non-split	O
66	B-Architecture
block	I-Architecture
contains	O
50	O
rows	O
;	O
each	O
row	O
has	O
two	O
(	O
E	O
)	O
or	O
four	O
(	O
M	O
)	O
or	O
six	O
(	O
A	O
)	O
&	O
(	O
B	O
)	O
columns	O
of	O
clips	O
that	O
are	O
electrically	O
bonded	O
.	O
</s>
<s>
The	O
25-pair	O
split	O
50	O
66	B-Architecture
block	I-Architecture
is	O
the	O
industry	O
standard	O
for	O
easy	O
termination	O
of	O
voice	O
cabling	O
,	O
and	O
is	O
a	O
standard	O
network	O
termination	O
by	O
telephone	O
companies	O
—	O
generally	O
on	O
commercial	O
properties	O
.	O
</s>
<s>
Circuit	O
pairs	O
are	O
connected	O
to	O
the	O
block	O
with	O
a	O
punch-down	B-Protocol
tool	I-Protocol
by	O
terminating	O
the	O
tip	O
wire	O
on	O
the	O
leftmost	O
slot	O
of	O
one	O
row	O
and	O
ring	O
wire	O
on	O
the	O
leftmost	O
slot	O
of	O
the	O
row	O
beneath	O
the	O
mating	O
tip	O
wire	O
.	O
</s>
<s>
An	O
orange	O
insulating	O
cover	O
attached	O
to	O
a	O
66	B-Architecture
block	I-Architecture
denotes	O
its	O
designation	O
as	O
a	O
demarcation	B-Architecture
point	I-Architecture
by	O
the	O
local	O
exchange	O
carrier	O
.	O
</s>
<s>
Modern	O
110	B-Device
blocks	I-Device
(	O
Originally	O
designated	O
as	O
88	O
blocks	O
)	O
have	O
largely	O
supplanted	O
66	B-Architecture
blocks	I-Architecture
for	O
new	O
commercial	O
installations	O
at	O
the	O
end	O
of	O
the	O
20th	O
century	O
,	O
as	O
the	O
capability	O
for	O
a	O
circuit	O
to	O
carry	O
digital	O
data	O
overlaid	O
its	O
ability	O
to	O
carry	O
analog	O
voice	O
conversations	O
.	O
</s>
<s>
110	B-Device
block	I-Device
termination	O
is	O
almost	O
always	O
compliant	O
with	O
Category	B-Protocol
5	I-Protocol
(	O
or	O
higher	O
)	O
and	O
therefore	O
capable	O
of	O
supporting	O
100MHz	O
(	O
or	O
faster	O
)	O
signaling	O
.	O
</s>
<s>
Compared	O
to	O
110	O
and	O
higher-density	O
wire	O
terminating	O
blocks	O
,	O
66	B-Architecture
blocks	I-Architecture
are	O
physically	O
large	O
;	O
and	O
because	O
of	O
their	O
maximum	O
16MHz	O
Category	B-Protocol
3	I-Protocol
signaling	O
compatibility	O
,	O
they	O
are	O
ill-suited	O
for	O
high	O
speed	O
(	O
faster	O
than	O
10BASE-T	B-Protocol
)	O
data	O
circuits	O
.	O
</s>
<s>
However	O
,	O
special	O
Category	B-Protocol
5e	I-Protocol
Certified	O
66	B-Architecture
blocks	I-Architecture
are	O
available	O
from	O
manufacturers	O
such	O
as	O
Siemon	O
which	O
meet	O
all	O
standards	O
for	O
Cat5e	B-Protocol
termination	O
.	O
</s>
<s>
Split	O
50	O
66	B-Architecture
blocks	I-Architecture
are	O
still	O
used	O
as	O
network	O
interface	O
blocks	O
in	O
distribution	O
frames	O
to	O
interconnect	O
circuits	O
with	O
bridging	O
clips	O
,	O
but	O
are	O
primarily	O
limited	O
to	O
narrowband	O
circuits	O
such	O
as	O
POTS/DSL	O
,	O
DS0	O
,	O
or	O
DS1	O
circuits	O
.	O
</s>
