We consider numeration systems with base β and −β, for quadratic Pisot numbers β and focus on comparingthe combinatorial structure of the sets Zβ and Z− β of numberswith integer expansion in base β, resp. − β. Our main result is the comparison of languagesof infinite words uβ andu−β coding the ordering of distances betweenconsecutive β- and (−β)-integers. It turns out that for a class of rootsβ ofx2 −mx − m, the languages coincide,while for other quadratic Pisot numbers the language of uβ can be identified onlywith the language of a morphic image of u− β. We also study thegroup structure of (−β)-integers.