The focus of a (contextualized) sentence S is the semantic component most important to the speaker. In more formal terms, it is the semantic component by which the assertion differs from the presuppositions, i.e. the component which is not contained in the presuppositions. It follows from the definition that there is an expression coding the focus; i.e. the focus cannot be implicit/zero.

Given a sentence, the question is: Which of its components is the expression of the focus? Preconditions for the analysis include:

• S is contextualized, i.e. part of a (natural) text.
• All phonological information is present, including, in particular, prosody (sentence stress, intonation, pauses).

Methods for identification of the focal expression of S include:

1. Transform S into a cleft-sentence, choosing one of its components as predicate of the main clause. If S still fits its context, that component is the focal expression.
2. Formulate a content interrogative sentence to which S is to be the answer. If your question - answer pair is felicitous, then the component of S replaced by the interrogative pronoun is the focal expression.