<s>
The	O
Les	B-Algorithm
Houches	I-Algorithm
Accords	I-Algorithm
are	O
agreements	O
between	O
particle	O
physicists	O
to	O
standardize	O
the	O
interface	O
between	O
the	O
matrix	O
element	O
programs	O
and	O
the	O
event	B-Algorithm
generators	I-Algorithm
used	O
to	O
calculate	O
different	O
quantities	O
.	O
</s>
<s>
However	O
,	O
there	O
are	O
several	O
programs	O
for	O
each	O
of	O
these	O
tasks	O
,	O
such	O
as	O
CompHEP	B-Application
and	O
MadGraph	O
to	O
generate	O
matrix	O
elements	O
,	O
and	O
PYTHIA	B-Language
and	O
HERWIG	O
for	O
event	O
generation	O
.	O
</s>
<s>
Depending	O
on	O
specific	O
properties	O
of	O
the	O
particle	O
decay	O
that	O
physicists	O
are	O
interested	O
in	O
,	O
they	O
may	O
desire	O
to	O
use	O
a	O
certain	O
program	O
for	O
these	O
tasks	O
,	O
but	O
before	O
the	O
Les	B-Algorithm
Houches	I-Algorithm
Accords	I-Algorithm
,	O
there	O
was	O
no	O
general	O
interface	O
for	O
communicating	O
between	O
the	O
programs	O
.	O
</s>
<s>
The	O
original	O
Accord	O
defined	O
a	O
programmatic	O
interface	O
for	O
transfer	O
of	O
event	O
information	O
,	O
in	O
terms	O
of	O
Fortran	B-Application
common	B-Application
blocks	I-Application
,	O
but	O
no	O
data	O
exchange	O
file	O
format	O
was	O
defined	O
until	O
2006	O
.	O
</s>
<s>
Events	O
that	O
conform	O
to	O
the	O
formats	O
described	O
in	O
the	O
Les	B-Algorithm
Houches	I-Algorithm
Accords	I-Algorithm
are	O
said	O
to	O
be	O
in	O
Les	O
Houches	O
Event	O
format	O
,	O
or	O
more	O
often	O
,	O
LHE	O
format	O
.	O
</s>
