▁A ▁gover ns ▁B ▁if ▁and ▁only ▁if ▁ ▁A ▁is ▁a ▁governor ▁and ▁ ▁A ▁m - commands ▁B ▁and ▁ ▁no ▁bar rier ▁interven es ▁between ▁A ▁and ▁B . ▁ ▁Govern ors ▁are ▁heads ▁of ▁the ▁lex ical ▁categories ▁( V , ▁N , ▁A , ▁P ) ▁and ▁t ensed ▁I ▁( T ). ▁A ▁m - commands ▁B ▁if ▁A ▁does ▁not ▁domin ate ▁B ▁and ▁B ▁does ▁not ▁domin ate ▁A ▁and ▁the ▁first ▁maximal ▁projection ▁of ▁A ▁domin ates ▁B , ▁where ▁the ▁maximal ▁projection ▁of ▁a ▁head ▁X ▁is ▁XP . ▁This ▁means ▁that ▁for ▁example ▁in ▁a ▁structure ▁like ▁the ▁following , ▁A ▁m - commands ▁B , ▁but ▁B ▁does ▁not ▁m - command ▁A : ▁ ▁In ▁addition , ▁bar rier ▁is ▁defined ▁as ▁follows : ▁A ▁bar rier ▁is ▁any ▁node ▁Z ▁such ▁that ▁ ▁Z ▁is ▁a ▁potential ▁governor ▁for ▁B ▁and ▁ ▁Z ▁c - commands ▁B ▁and ▁ ▁Z ▁does ▁not ▁c - command ▁A ▁ ▁The ▁government ▁relation ▁makes ▁case ▁assignment ▁un ambigu ous . ▁The ▁tree ▁diagram ▁below ▁illustr ates ▁how ▁D P s ▁are ▁govern ed ▁and ▁assigned ▁case ▁by ▁their ▁gover ning ▁heads : ▁ ▁Another ▁important ▁application ▁of ▁the ▁government ▁relation ▁const ra ins ▁the ▁occurrence ▁and ▁identity ▁of ▁traces ▁as ▁the ▁Em pty ▁Category ▁Princi ple ▁requires ▁them ▁to ▁be ▁properly ▁govern ed . ▁ ▁Binding ▁Binding ▁can ▁be ▁defined ▁as ▁follows : ▁▁ ▁An ▁element ▁α ▁bind s ▁an ▁element ▁