Redistribution is the process, used in many Commonwealth countries, by which electoral districts are added, removed, or otherwise changed. Redistribution is a form of boundary delimitation that changes electoral district boundaries, usually in response to periodic census results. Redistribution is required by law or constitution at least every decade in most representative democracy systems that use first-past-the-post or similar electoral systems to prevent geographic malapportionment. The act of manipulation of electoral districts to favour a candidate or party is called gerrymandering.