<s>
A	O
constraint	B-Algorithm
in	O
computer-aided	B-Application
design	I-Application
(	O
CAD	B-Application
)	O
software	O
is	O
a	O
limitation	O
or	O
restriction	O
imposed	O
by	O
a	O
designer	O
or	O
an	O
engineer	O
upon	O
geometric	O
properties	O
of	O
an	O
entity	O
of	O
a	O
design	O
model	O
that	O
maintains	O
its	O
structure	O
as	O
the	O
model	O
is	O
manipulated	O
.	O
</s>
<s>
The	O
exact	O
terminology	O
,	O
however	O
,	O
may	O
vary	O
depending	O
on	O
a	O
CAD	B-Application
program	I-Application
vendor	O
.	O
</s>
<s>
Constraints	O
are	O
widely	O
employed	O
in	O
CAD	B-Application
software	I-Application
for	O
solid	O
modeling	O
,	O
computer-aided	B-Architecture
architectural	I-Architecture
design	I-Architecture
such	O
as	O
building	B-Architecture
information	I-Architecture
modeling	I-Architecture
,	O
computer-aided	O
engineering	O
,	O
assembly	O
modeling	O
,	O
and	O
other	O
CAD	B-Application
subfields	O
.	O
</s>
<s>
A	O
constraint	B-Algorithm
may	O
be	O
specified	O
for	O
two	O
or	O
more	O
entities	O
at	O
once	O
.	O
</s>
<s>
Moreover	O
,	O
the	O
constraint	B-Algorithm
may	O
be	O
applied	O
to	O
solid	O
models	O
to	O
be	O
locked	O
or	O
fixed	O
in	O
a	O
specified	O
space	O
.	O
</s>
<s>
It	O
is	O
derived	O
from	O
ideas	O
employed	O
in	O
Sketchpad	B-Application
system	O
made	O
in	O
1963	O
.	O
</s>
<s>
In	O
his	O
work	O
he	O
argued	O
that	O
the	O
usefulness	O
of	O
a	O
technical	B-Application
drawing	I-Application
made	O
by	O
a	O
computer	O
program	O
relied	O
on	O
their	O
structured	O
nature	O
.	O
</s>
<s>
These	O
ideas	O
were	O
integrated	O
into	O
a	O
CAD	B-Application
system	O
that	O
maintained	O
this	O
structure	O
as	O
a	O
designer	O
manipulated	O
geometric	O
model	O
.	O
</s>
<s>
In	O
the	O
80s	O
,	O
a	O
more	O
generalized	O
constraint-based	O
programming	O
language	O
approach	O
emerged	O
and	O
found	O
some	O
application	O
in	O
CAD	B-Application
software	I-Application
.	O
</s>
<s>
More	O
advanced	O
2D/3D	O
CAD	B-Application
systems	O
may	O
allow	O
application	O
of	O
mathematical	O
relationships	O
between	O
constraints	O
that	O
help	O
to	O
save	O
time	O
on	O
reshaping	O
a	O
model	O
.	O
</s>
<s>
Implementation	O
of	O
constraints	O
functionality	O
vary	O
with	O
given	O
CAD	B-Application
system	O
and	O
may	O
respond	O
differently	O
to	O
how	O
user	O
applies	O
them	O
.	O
</s>
<s>
Constraint	B-Algorithm
solver	O
is	O
a	O
dedicated	O
software	O
that	O
calculates	O
positions	O
of	O
points	O
of	O
the	O
2D	O
sketch	O
based	O
on	O
geometric	O
constraint	B-Algorithm
specified	O
by	O
the	O
user	O
.	O
</s>
<s>
The	O
purpose	O
of	O
the	O
constraint	B-Algorithm
solver	O
is	O
to	O
find	O
all	O
points	O
 '	O
positions	O
with	O
respect	O
to	O
the	O
said	O
constraints	O
.	O
</s>
<s>
Ideally	O
,	O
a	O
rod	O
will	O
need	O
to	O
be	O
concentric	O
to	O
a	O
hole	O
drilled	O
through	O
the	O
plate	O
where	O
it	O
will	O
be	O
inserted	O
,	O
so	O
the	O
constraint	B-Algorithm
"	O
concentric	O
"	O
guarantees	O
that	O
the	O
diameter	O
of	O
the	O
rod	O
and	O
the	O
diameter	O
of	O
the	O
hole	O
maintain	O
a	O
common	O
centerline	O
,	O
thus	O
"	O
locking	O
"	O
the	O
manner	O
the	O
rod	O
relates	O
to	O
the	O
hole	O
in	O
the	O
plate	O
;	O
this	O
means	O
that	O
the	O
rod	O
could	O
still	O
slide	O
on	O
either	O
direction	O
since	O
the	O
position	O
of	O
its	O
ends	O
has	O
not	O
been	O
limited	O
.	O
</s>
