Currently,alotofeffortisbeingputondesigningcomplexdetectors.Anumberofsimulationandreconstructionframeworksandapplicationshavebeendevelopedwiththeaimtomakethisjobeasier.Averyimportantroleinthisactivityisplayedbythegeometrydescriptionofthedetectorapparatuslayoutanditsworkingenvironment.However,norealcommonapproachtorepresentgeometrydataisavailableandsuchdatacanbefoundinvariousformsstartingfromcustomsemi-structuredtextfiles,sourcecode(C/C++/FORTRAN),toXMLanddatabasesolutions.TheXML(ExtensibleMarkupLanguage)hasproventoprovideaninterestingapproachfordescribingdetectorgeometries,withseveraldifferentbutincompatibleXML-basedsolutionsexisting.Therefore,interoperabilityandgeometrydataexchangeamongdifferentframeworksisnotpossibleatpresent.Thisarticleintroducesamarkuplanguageforgeometrydescriptions.Itsaimistodefineacommonapproachforsharingandexchangingofgeometrydescriptiondata.ItsrequirementsanddesignhavebeendrivenbyexperienceanduserfeedbackfromexistingprojectswhichhavetheirgeometrydescriptioninXML.
