<s>
In	O
computer	O
science	O
tree	B-Application
data	I-Application
structures	I-Application
,	O
an	O
X-tree	B-Data_Structure
(	O
for	O
eXtended	O
node	O
tree	O
)	O
is	O
an	O
index	O
tree	O
structure	O
based	O
on	O
the	O
R-tree	B-Library
used	O
for	O
storing	O
data	O
in	O
many	O
dimensions	O
.	O
</s>
<s>
It	O
appeared	O
in	O
1996	O
,	O
and	O
differs	O
from	O
R-trees	B-Library
(	O
1984	O
)	O
,	O
R+	B-Data_Structure
-trees	I-Data_Structure
(	O
1987	O
)	O
and	O
R*	B-Data_Structure
-trees	I-Data_Structure
(	O
1990	O
)	O
because	O
it	O
emphasizes	O
prevention	O
of	O
overlap	O
in	O
the	O
bounding	O
boxes	O
,	O
which	O
increasingly	O
becomes	O
a	O
problem	O
in	O
high	O
dimensions	O
.	O
</s>
<s>
The	O
X-tree	B-Data_Structure
consists	O
of	O
three	O
different	O
types	O
of	O
nodes	O
—	O
data	O
nodes	O
,	O
normal	O
directory	O
nodes	O
and	O
supernodes	O
.	O
</s>
<s>
The	O
data	O
nodes	O
of	O
the	O
X-tree	B-Data_Structure
contain	O
rectilinear	O
minimum	B-Algorithm
bounding	I-Algorithm
rectangles	I-Algorithm
(	O
MBRs	O
)	O
together	O
with	O
pointers	O
to	O
the	O
actual	O
data	O
objects	O
,	O
and	O
the	O
directory	O
nodes	O
contain	O
MBRs	O
together	O
with	O
pointers	O
to	O
sub-MBRs	O
.	O
</s>
