Subject and predicate are both grammatical functions. The predicate is realized by a verb phrase, and in the most common case, the subject is realized by a noun phrase. Notice that in the diagram above, we indicate both the grammatical form (the phrase type) and the function. The two are separated by a colon. Thus our notation follows the pattern form: function.
One important point to note about subjects is that they frequently consist of more than one word.
Many students are taught in grade school to identify the word senator alone as the subject. However, notice that senator is merely the head noun of the subject. The determiner the and the prepositional phrase from California are also part of the subject. In other words, subjects and predicates, along with other grammatical functions we will encounter later, are functions of phrases, not of individual words. As we noted above, however, those phrases may consist of only one word from time to time.
 Some theories of grammar do not mark functions as a matter of principal. Such theories attempt to give the most parsimonious account possible, and in this way of looking at things, grammatical roles such as subject are predictable from the structure. Although it may be redundant to mark such roles, we do so here for pedagogical reasons. We are primarily interested in describing all the relevant grammatical features in a way that is relatively easy to interpret, and to that end, we will tolerate a certain amount of redundancy.