The traditional beer mug used at Oktoberfest is called a Maß.
Strictly speaking, Maß refers to the one-litre serving or measure, while the handled vessel is a Maßkrug. In everyday English and Bavarian festival speech, however, the mug itself is commonly shortened to “a Maß,” making the stored answer the standard quiz answer.
The modern Maßkrug is usually made of clear glass, allowing drinkers and servers to check its calibration line. Stoneware versions also exist, but a vessel is properly called a Maßkrug only when it holds the regulation measure. The word is associated especially with Bavaria, beer gardens, beer halls and Munich’s Oktoberfest.
A common mix-up is calling every German beer mug a stein. “Stein” is a broad English term, while Maß identifies the regulated one-litre serving tradition. The mug is slightly larger than one litre so there is room above the fill line for foam.