<s>
In	O
software	O
object-oriented	B-Application
design	I-Application
,	O
a	O
layer	O
is	O
a	O
group	O
of	O
classes	O
that	O
have	O
the	O
same	O
set	O
of	O
link-time	O
module	B-Architecture
dependencies	B-Application
to	O
other	O
modules	B-Architecture
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
layer	O
is	O
a	O
group	O
of	O
reusable	B-Application
components	O
that	O
are	O
reusable	B-Application
in	O
similar	O
circumstances	O
.	O
</s>
<s>
In	O
programming	O
languages	O
,	O
the	O
layer	O
distinction	O
is	O
often	O
expressed	O
as	O
"	O
import	O
"	O
dependencies	B-Application
between	O
software	O
modules	B-Architecture
.	O
</s>
<s>
Layers	O
are	O
often	O
arranged	O
in	O
a	O
tree-form	O
hierarchy	O
,	O
with	O
dependency	B-Application
relationships	O
as	O
links	O
between	O
the	O
layers	O
.	O
</s>
<s>
Dependency	B-Application
relationships	O
between	O
layers	O
are	O
often	O
either	O
inheritance	B-Language
,	O
composition	B-Language
or	O
aggregation	O
relationships	O
,	O
but	O
other	O
kinds	O
of	O
dependencies	B-Application
can	O
also	O
be	O
used	O
.	O
</s>
