Altbier (often abbreviated to Alt) is the name given to a form of German top-fermenting beer that originated in Westphalia and spread to parts of the Rhineland later.
The name Altbier, which literally means old [style] beer, refers to the pre-lager brewing method of using a warm top-fermenting yeast like British pale ales. Over time the Alt yeast adjusted to lower temperatures, and the Alt brewers would store or lager the beer after fermentation, leading to a cleaner, crisper beer than is the norm for an ale.
The Bavarian Reinheitsgebot (beer purity law; literally "purity order") of 1516 was drawn up to ensure the production of decent-quality beer; however, this decree did not affect brewers of the Rhineland. As such, the brewing traditions in this region developed slightly differently. For example, brewing during the summer was illegal in Bavaria, but the cooler climate of the Rhineland allowed Alt brewers to brew all year long and to experiment with storing fermented beer in cool caves and cellars.
The name "altbier" first appeared in the 1800s to differentiate the beers of Düsseldorf from the new pale lager that was gaining a hold on Germany. Brewers in Düsseldorf used the pale malts that were used for the modern pale lagers, but retained the old ("alt") method of using warm fermenting yeasts.
The first brewery to use the name Alt was Schumacher which opened in 1838. The founder, Mathias Schumacher, allowed the beer to mature in cool conditions in wooden casks for longer than normal, and laid the foundation for the modern alt beer - a dark, lagered top-fermented beer. The result is a beer that has some of the lean, dryness of a lager, with the fruity notes of an ale.