A2

A2 is a designation used in several different contexts, often without relation to each other. The term appears in publishing, biology, mathematics, transportation, and language frameworks, among other fields. This article summarizes several common uses.

- A2 paper size: In the ISO 216 standard, A2 is a paper size measuring 420 x 594 millimeters. It is part of the A-series, which is based on a 1:√2 aspect ratio and whose sizes halve in area when scaled. A2 is commonly used for posters, diagrams, and architectural drawings.

- A2 milk: A2 milk refers to milk from cows that predominantly produce the A2 variant of beta-casein protein, rather than the A1 variant. Producers market A2 milk as potentially easier to digest for some people, though scientific consensus on health benefits remains inconclusive.

- A2 in mathematics and physics: In Lie theory, A2 denotes a simple Lie algebra of type A (specifically sl3(C)). The designation relates to the root system A2 and plays a role in representation theory and geometry. In other contexts, A2 may appear as a label for specific coordinate systems, lattices, or symmetry structures.

- A2 road designations: Across several countries, A2 denotes a major road or motorway. For example, in the United Kingdom the A2 road runs from London to the Channel port of Dover, while in Germany the A2 Autobahn connects the Dutch border to the Polish border. Similar conventions appear in other nations, with A2 typically indicating significant regional routes.

- CEFR language level: In the Common European Framework of Reference for Languages, A2 is the second level of the “A” (basic) tier. It describes basic ability to communicate in simple, familiar situations, with limited vocabulary and simple sentence structures.

A2 thus functions as a cross-domain label rather than a single concept, with each usage rooted in its specific field.