vastastikusel

Vastastikusel is an Estonian inflected form of the adjective vastastik, meaning reciprocal or mutual. It is used to describe actions, relations, or arrangements that occur on a reciprocal basis. The form typically appears in formal or academic text and is combined with a noun in the adessive case to indicate the basis or manner of an arrangement, for example vastastikusel alusel ("on a mutual basis").

In grammar, vastastik is the base adjective, and vastastikusel represents one of its case-inflected forms. As with other Estonian adjectives, its ending agrees with the noun it modifies and the syntactic role of the phrase. Thus vastastikusel is not a distinct concept by itself but a grammatical form used to express reciprocity within a noun phrase.

Common contexts for its use include descriptions of cooperation, exchange, or ethical norms, where mutual obligations or benefits are emphasized. For instance, sentences such as "Koostöö toimub vastastikusel alusel" (Cooperation operates on a mutual basis) illustrate how the phrase signals reciprocity in practical arrangements. Another example might describe agreement terms or policies that are based on reciprocal principles.

Notes of usage: vastastikusel tends to occur in more formal registers, policy language, or scholarly writing. It is part of a broader discourse on reciprocity in Estonian, alongside related terms such as vastastikune (mutual) and phrases denoting reciprocal arrangements. In summary, vastastikusel functions as a grammatical tool to encode reciprocity within Estonian noun phrases.