Since initially presented in the 1982 Department of Justice Horizontal Merger Guidelines, market definition has been adopted nearly worldwide as a framework to see if a merger would substantially lessen competition. This framework is useful for addressing the similarly counterfactual question of...

The 'line of business' restrictions imposed upon the local Bell operating companies after their divestiture from AT&T were justified as necessary to prevent the undue exercise of market power in local exchange service through cross-subsidization or discriminatory tactics. However, these...