A primary employer is one involved in a dispute, and a neutral employer is one affected by the picketing a of the primary's employees. The laws governing common situs picketing in construction were established in the Denver Building Trades Council cases. Picketing began when it was learned the primary contractor on the site had employed a nonunion subcontractor. The picketing was aimed at forcing the general contractor to drop the subcontractor. When picketing began, all other union workers refused to cross the picket line. The primary contractor maintained the dispute was with the subcontractor and general picketing of the site was an illegal secondary boycott designed to force neutral employers not to do business with the subcontractor. The court's ruling forbids construction unions to picket sites to force a primary employer to cease doing business with a nonunion subcontractor. The usual practice at construction sites to establish reserved gates for each employer. Primary-dispute pickets may then patrol only their employer's gate