Complements and adjuncts are clause components which depend on another component which is their head. The distinction applies primarily to naked and cased noun phrases and adpositional phrases, secondarily to other kinds of syntagmas with the same distribution as these. It is based on grammatical properties of such dependents, viz.:
- the grammatical subcategory of the dependent (mass vs. count, ...)
- the syntactic function of the dependent (subject, direct object ...)
- the morphological marking associated with this syntactic function (case, adposition ...)
- the omissibility of the dependent (optional vs. obligatory).
A complement is a dependent whose grammatical properties are determined by its head.
An adjunct is a dependent whose grammatical properties are independent from its head.
As an example, consider the ambiguity of John decided on the boat:
- It can mean that the boat is the object of John's decision. The prepositional phrase on the boat is then a complement of the verb decide. One of its properties determined by the verb decide is the preposition on.
- It can mean that John passed the decision while being on the boat. The prepositional phrase is then an adjunct. The preposition then does not need to be on; it could be under or any semantically suitable preposition; and the entire adjunct could be replaced by a different one or by zero.
- A complement is said to be governed by its head and to be part of its valency.
- An adjunct is said to modify its head and to be independent of its valency.