Technically, authentic Oktoberfestbier is brewed only through the breweries within the city limits of Munich. These are also the only breweries permissible to participate in the annual Munich Oktoberfest. All other beers brewed to emulate the original must be labeled Oktoberfest - style beer. In malevolence of their name, Oktoberfest beers are not brewed in the fall. They are really an outgrowth of the traditional, strong spring brews, called March beers/M'rzen, that were put aside in ice-filled caves or cellars for summer consumption. The left-over M'rzen was usually finished off in Okotober, when the fresh beers made among the grain and hops from the new harvest season needed to be put into casks. Oktoberfestbiers, therefore, are always well-aged, occasionally for three to four months.
