Dependency Syntax and Universal Dependencies › What Makes a Head?

The single-head axiom tells you that each token has exactly one head. It does not tell you which head. That is a linguistic question, and linguists have developed several tests. The tests are diagnostics rather than mathematical definitions. They can point toward a head, but they do not always agree, and when they disagree, an annotation project has to adopt a convention.

Several diagnostics can motivate a head choice, but none replaces a stated annotation convention:

• Distribution. A head often helps determine where the phrase can occur. The noun dog is central to the nominal expression the big dog. This is a diagnostic, not a proof that all theories must choose the same head.
• Omission. Removing big leaves the dog, but removing dog does not preserve that same ordinary noun-phrase reading. Ellipsis and nominalized adjectives can license exceptions, as in the rich.
• Selection and agreement. A predicate constrains its argument pattern, while agreement can reveal dependencies in languages that mark it. English big does not agree with dog in number, and him is an objective pronoun form rather than a distinct dative inflection.
• Meaning. In big dog, the noun identifies the entity being described and the adjective contributes a property. A different construction can make semantic and syntactic headhood diverge, so meaning alone is not a universal head-selection algorithm.

Moving a whole phrase is evidence about constituency, rather than a test that uniquely identifies its head. In the rich are getting richer, UD treats rich as the head of the nominal expression and the as a determiner dependent. Such examples show why an annotation scheme must explain its conventions instead of treating head choice as self-evident.

UD separates word-level morphology from syntax and generally places content words at the center of dependency structures. Function words normally depend on the related content word; the guidelines include exceptions. This matters because many syntactic theories treat determiners, auxiliaries, and complementizers as heads of their own functional projections. UD does not. The lexicalist choice keeps the basic tree close to the words that carry the main lexical content, and it avoids introducing a large amount of abstract functional structure into every sentence.

In the examples below, UD uses the following head-dependent choices:

• Determiners are dependents. In the dog, dog is head and the is a det dependent.
• Auxiliaries are dependents. In she will go, go is head and will is an aux dependent.
• Copulas are dependents. In she is a doctor, doctor is head and is is a cop dependent.
• Adpositions are dependents. In the dog of the neighbor, of is a case dependent of neighbor.
• Complementizers are dependents. In that she left surprised him, left is head of the embedded clause, and that is a mark dependent.

The rationale is cross-linguistic. A grammatical distinction expressed by an independent function word in one language may be expressed by inflection in another. Keeping content words as the syntactic backbone increases parallelism without inventing a dependency node for every suffix. Bound morphology remains a property of its word, such as Case=Ins; a separate adposition can appear as a case dependent. See the UD syntax principles and case guidance.