A sentence diagram is a pictorial representation of the grammatical structure of a sentence. The term "sentence diagram" is used more when teaching written language, where sentences are diagrammed. The term "parse tree" is used in linguistics (especially computational linguistics), where sentences are parsed. Both show structure of sentences. The model shows the relations between words and the nature of sentence structure and can be used as a tool to help recognize which potential sentence is actually a sentence.
Most methods of diagramming in pedagogy are based on the work of Alonzo Reed and Brainerd Kellogg in their book Higher Lessons in English, first published in 1877, though the method has been updated with recent understanding of grammar. Reed and Kellogg were preceded, and their work probably informed, by W. S. Clark, who published his "balloon" method of depicting grammar in his 1847 book A Practical Grammar: In Which Words, Phrases & Sentences are Classified According to Their Offices and Their Various Relationships to Each Another.
Some schoolteachers continue to use the Reed-Kellogg system in teaching grammar, but others have discouraged it in favor of more modern tree diagrams. However, these modern tree structures draw on techniques that were already present in Reed-Kellogg diagrams. Reed and Kellogg defend their system in the preface to their grammar:
The Objections to the Diagram.--The fact that the pictorial diagram groups the parts of a sentence according to their offices and relations, and not in the order of speech, has been spoken of as a fault. It is, on the contrary, a merit, for it teaches the pupil to look through the literary order and discover the logical order. He thus learns what the literary order really is, and sees that this may be varied indefinitely, so long as the logical relations are kept clear.
The assertion that correct diagrams can be made mechanically is not borne out by the facts. It is easier to avoid precision in oral analysis than in written. The diagram drives the pupil to a most searching examination of the sentence, brings him face to face with every difficulty, and compels a decision on every point.
These statements bear witness to the fact that Reed-Kellogg diagrams abstract away from actual word order in order to focus more intently on how words in sentences function and relate to each other.
Simple sentences in the Reed-Kellogg system are diagrammed according to these forms:
The diagram of a simple sentence begins with a horizontal line called the base. The subject is written on the left, the predicate on the right, separated by a vertical bar which extends through the base. The predicate must contain a verb, and the verb either requires other sentence elements to complete the predicate, permits them to do so, or precludes them from doing so. The verb and its object, when present, are separated by a line that ends at the baseline. If the object is a direct object, the line is vertical. If the object is a predicate noun or adjective, the line looks like a backslash, \, sloping toward the subject.
Modifiers of the subject, predicate, or object are placed below the base line:
Modifiers, such as adjectives (including articles) and adverbs, are placed on slanted lines below the word they modify. Prepositional phrases are also placed beneath the word they modify; the preposition goes on a slanted line and the slanted line leads to a horizontal line on which the object of the preposition is placed.
The connections to modern principles for constructing parse trees are present in the Reed-Kellogg diagrams, although Reed and Kellogg understood such principles only implicitly. The principles are now regarded as the constituency relation of phrase structure grammars and the dependency relation of dependency grammars. These two relations are illustrated here adjacent to each other for comparison:
Constituency is a one-to-one-or-more relation; every word in the sentence corresponds to one or more nodes in the tree diagram. Dependency, in contrast, is a one-to-one relation; every word in the sentence corresponds to exactly one node in the tree diagram. Both parse trees employ the convention where the category acronyms (e.g. N, NP, V, VP) are used as the labels on the nodes in the tree. The one-to-one-or-more constituency relation is capable of increasing the amount of sentence structure to the upper limits of what is possible. The result can be very "tall" trees, such as those associated with X-bar theory. Both constituency-based and dependency-based theories of grammar have established traditions.
Reed-Kellogg diagrams employ both of these modern tree generating relations. The constituency relation is present in the Reed-Kellogg diagrams insofar as subject, verb, object, and/or predicate are placed equi-level on the horizontal base line of the sentence and divided by a vertical or slanted line. In a Reed-Kellogg diagram, the vertical dividing line that crosses the base line corresponds to the binary division in the constituency-based tree (S -> NP + VP), and the second vertical dividing line that does not cross the baseline (between verb and object) corresponds to the binary division of VP into verb and direct object (VP -> V + NP). Thus the vertical and slanting lines that cross or rest on the baseline correspond to the constituency relation. The dependency relation, in contrast, is present insofar as modifiers dangle off of or appear below the words that they modify.
One can render Reed-Kellogg diagrams according to modern tree conventions. When one does so, the result is a hybrid dependency-constituency tree. The Reed-Kellogg diagrams above appear as the following trees:
A mixing of labeling conventions (i.e. category label vs. actual word) helps draw attention to the presence of both constituency and dependency. The S and VP in these trees mark the constituency relation and the words themselves mark the dependency relation. A major difference between these hybrid trees and the Reed-Kellogg diagrams, however, is that the hybrid trees encode actual word order, whereas the Reed-Kellogg diagrams are abstracting away from actual word order in order to focus more on function.
A sentence may also be broken down by functional parts: subject, object, adverbial, verb (predicator). The subject is the owner of an action, the verb represents the action, the object represents the recipient of the action, and the adverbial qualifies the action. The various parts can be phrases rather than individual words.