Wechselpräposition

Wechselpräpositionen, or two-way prepositions, are a class of German prepositions that can govern either the accusative or the dative case depending on the context. The most common Wechselpräpositionen are an, auf, hinter, in, neben, über, unter, vor and zwischen. They express spatial relations and can also appear in figurative uses.

The case of a Wechselpräposition depends on whether there is movement toward a destination or a fixed position. If the action involves movement toward something (a change of location), the object is in the accusative case. If there is no movement and the reference is a location or position, the object is in the dative case. A practical rule is to answer wohin? with the accusative and wo? with the dative.

Examples:

Movement: Ich gehe in das Haus. Ich lege das Buch auf den Tisch. In both sentences the preposition expresses direction toward a new location, so the accusative is used.

Location: Das Kind sitzt in dem Zimmer. Der Schlüssel liegt auf dem Tisch. Here the prepositions indicate position, so the dative is used.

Contractions with definite articles are common:

an + dem → am, in + dem → im, an + das → ans, in + das → ins, auf + das → aufs. Note that auf dem is usually not contracted in written standard German.

Wechselpräpositionen require attention to case and article form, as the meaning changes with direction versus location. They are fundamental for expressing spatial relations and movement in German.