<s>
A	O
tree	B-Data_Structure
structure	I-Data_Structure
,	O
tree	O
diagram	O
,	O
or	O
tree	O
model	O
is	O
a	O
way	O
of	O
representing	O
the	O
hierarchical	O
nature	O
of	O
a	O
structure	O
in	O
a	O
graphical	O
form	O
.	O
</s>
<s>
It	O
is	O
named	O
a	O
"	O
tree	B-Data_Structure
structure	I-Data_Structure
"	O
because	O
the	O
classic	O
representation	O
resembles	O
a	O
tree	O
,	O
although	O
the	O
chart	O
is	O
generally	O
upside	O
down	O
compared	O
to	O
a	O
biological	O
tree	O
,	O
with	O
the	O
"	O
stem	O
"	O
at	O
the	O
top	O
and	O
the	O
"	O
leaves	O
"	O
at	O
the	O
bottom	O
.	O
</s>
<s>
A	O
tree	B-Data_Structure
structure	I-Data_Structure
is	O
conceptual	O
,	O
and	O
appears	O
in	O
several	O
forms	O
.	O
</s>
<s>
For	O
a	O
discussion	O
of	O
tree	B-Data_Structure
structures	I-Data_Structure
in	O
specific	O
fields	O
,	O
see	O
Tree	O
(	O
data	O
structure	O
)	O
for	O
computer	B-General_Concept
science	I-General_Concept
;	O
insofar	O
as	O
it	O
relates	O
to	O
graph	O
theory	O
,	O
see	O
tree	O
(	O
graph	O
theory	O
)	O
or	O
tree	O
(	O
set	O
theory	O
)	O
.	O
</s>
<s>
Nodes	O
without	O
children	O
are	O
called	O
leaf	B-Application
nodes	I-Application
,	O
"	O
end-nodes	O
"	O
,	O
or	O
"	O
leaves	O
"	O
.	O
</s>
<s>
Every	O
finite	O
tree	B-Data_Structure
structure	I-Data_Structure
has	O
a	O
member	O
that	O
has	O
no	O
superior	O
.	O
</s>
<s>
This	O
member	O
is	O
called	O
the	O
"	O
root	O
"	O
or	O
root	B-Application
node	I-Application
.	O
</s>
<s>
The	O
root	O
is	O
the	O
starting	O
node	B-Data_Structure
.	O
</s>
<s>
But	O
the	O
converse	O
is	O
not	O
true	O
:	O
infinite	O
tree	B-Data_Structure
structures	I-Data_Structure
may	O
or	O
may	O
not	O
have	O
a	O
root	B-Application
node	I-Application
.	O
</s>
<s>
A	O
node	B-Data_Structure
's	O
"	O
parent	O
"	O
is	O
a	O
node	B-Data_Structure
one	O
step	O
higher	O
in	O
the	O
hierarchy	O
(	O
i.e.	O
</s>
<s>
closer	O
to	O
the	O
root	B-Application
node	I-Application
)	O
and	O
lying	O
on	O
the	O
same	O
branch	O
.	O
</s>
<s>
"	O
Sibling	O
"	O
(	O
"	O
brother	O
"	O
or	O
"	O
sister	O
"	O
)	O
nodes	O
share	O
the	O
same	O
parent	B-Application
node	I-Application
.	O
</s>
<s>
A	O
node	B-Data_Structure
's	O
"	O
uncles	O
"	O
(	O
sometimes	O
"	O
ommers	O
"	O
)	O
are	O
siblings	O
of	O
that	O
node	B-Data_Structure
's	O
parent	O
.	O
</s>
<s>
A	O
node	B-Data_Structure
that	O
is	O
connected	O
to	O
all	O
lower-level	O
nodes	O
is	O
called	O
an	O
"	O
ancestor	O
"	O
.	O
</s>
<s>
The	O
connected	O
lower-level	O
nodes	O
are	O
"	O
descendants	O
"	O
of	O
the	O
ancestor	B-Application
node	I-Application
.	O
</s>
<s>
Finally	O
,	O
"	O
science	O
"	O
,	O
"	O
art	O
"	O
and	O
"	O
craft	O
"	O
,	O
as	O
leaves	O
,	O
are	O
ancestors	O
of	O
no	O
other	O
node	B-Data_Structure
.	O
</s>
<s>
Tree	B-Data_Structure
structures	I-Data_Structure
can	O
depict	O
all	O
kinds	O
of	O
taxonomic	O
knowledge	O
,	O
such	O
as	O
family	O
trees	O
,	O
the	O
biological	O
evolutionary	O
tree	O
,	O
the	O
evolutionary	O
tree	O
of	O
a	O
language	O
family	O
,	O
the	O
grammatical	O
structure	O
of	O
a	O
language	O
(	O
a	O
key	O
example	O
being	O
S	O
→	O
NP	O
VP	O
,	O
meaning	O
a	O
sentence	O
is	O
a	O
noun	O
phrase	O
and	O
a	O
verb	O
phrase	O
,	O
with	O
each	O
in	O
turn	O
having	O
other	O
components	O
which	O
have	O
other	O
components	O
)	O
,	O
the	O
way	O
web	O
pages	O
are	O
logically	O
ordered	O
in	O
a	O
web	O
site	O
,	O
mathematical	B-Data_Structure
trees	I-Data_Structure
of	I-Data_Structure
integer	I-Data_Structure
sets	I-Data_Structure
,	O
et	O
cetera	O
.	O
</s>
<s>
The	B-Application
Oxford	I-Application
English	I-Application
Dictionary	I-Application
records	O
use	O
of	O
both	O
the	O
terms	O
"	O
tree	B-Data_Structure
structure	I-Data_Structure
"	O
and	O
"	O
tree-diagram	O
"	O
from	O
1965	O
in	O
Noam	O
Chomsky	O
's	O
Aspects	O
of	O
the	O
Theory	O
of	O
Syntax	O
.	O
</s>
<s>
In	O
a	O
tree	B-Data_Structure
structure	I-Data_Structure
there	O
is	O
one	O
and	O
only	O
one	O
path	O
from	O
any	O
point	O
to	O
any	O
other	O
point	O
.	O
</s>
<s>
Computer	B-General_Concept
science	I-General_Concept
uses	O
tree	B-Data_Structure
structures	I-Data_Structure
extensively	O
(	O
see	O
Tree	O
(	O
data	O
structure	O
)	O
and	O
telecommunications	O
.	O
)	O
</s>
<s>
Document	B-General_Concept
Object	I-General_Concept
Model	I-General_Concept
's	O
logical	O
structure	O
,	O
Yahoo	B-Application
!	I-Application
</s>
<s>
Computer	B-General_Concept
Science	I-General_Concept
:	O
</s>
<s>
There	O
are	O
many	O
ways	O
of	O
visually	O
representing	O
tree	B-Data_Structure
structures	I-Data_Structure
.	O
</s>
<s>
Classical	O
node-link	O
diagrams	O
,	O
that	O
connect	O
nodes	O
together	O
with	O
line	O
segments	O
:	O
</s>
<s>
Nested	O
sets	O
that	O
use	O
enclosure	O
or	O
containment	O
to	O
show	O
parenthood	O
;	O
examples	O
include	O
TreeMaps	B-Application
,	O
fractal	O
maps	O
,	O
and	O
Euler	B-Application
diagrams	I-Application
:	O
</s>
<s>
Trees	O
can	O
also	O
be	O
represented	B-Data_Structure
radially	I-Data_Structure
:	O
</s>
