Q. Please explain how to diagram a sentence.
A. First spread the sentence out on a clean, flat surface, such as an ironing board. Then, using a sharp pencil or X-Acto knife, locate the 'predicate,' which indicates where the action has taken place and is usually located directly behind the gills. For example, in the sentence: 'LaMont never would of bit a forest ranger,' the action probably took place in a forest. Thus your diagram would be shaped like a little tree with branches sticking out of it to indicate the locations of the various particles of speech, such as your gerunds, proverbs, adjutants, etc.
—Dave Barry, Ask Mr. Language Person

Grammarians like diagrams. You may have been compelled to draw something this in school:

from Reed and Kellogg, Graded Lessons in English, p. 60

Sometimes, students spend so much time drawing diagrams that they come to think of them as all there is to grammar. So what's the point of diagrams? Diagrams show you the constituency of sentences visually. As we have said above, constituency is one of the central issues of syntax, so diagrams make important assertions about language, but keep in mind that diagrams are only a tool, a method of showing what you understand about sentence structure that other people will be able to apprehend rapidly.

Diagrams drawn with the method illustrated above are known as Reed-Kellogg diagrams, after the authors who developed this system in the 1860s. Although such diagrams are frequently encountered in junior high and high school textbooks, they are rarely found outside the schoolroom. Of course, Reed-Kellogg diagrams are meant to be pedagogical, so that limitation is not necessarily a bad thing. They do capture a number of important features of sentence structure in a clear visual layout. In the diagram above, for example, we can quickly appreciate the core of the sentence and how the other elements relate to that core.

For the purposes of giving a truly accurate structural view of a sentence, however, Reed-Kellogg diagrams have many limitations. One drawback is that to understand these diagrams, you need to learn the significance of a relatively wide variety of different symbols. In this diagram alone we have thick horizontal line, thin horizontal lines, different kinds of slanting lines, and a dotted line. And there are a number of other symbols for structures not found in this example. More significantly, notice, in the diagram above, that determiners like the, adverbs like very, and prepositions like of are all indicated in the same way: by writing them on a slanting line. In other words, no distinction is made among these three very different word classes. As we develop our account of English syntax, we will see other ways in which Reed-Kellogg diagrams give a misleading picture of English syntax.

Linguists favor a different method for representing structure, known as a "tree diagram." You have already seen several of these tree diagrams, but we have not stopped to look closely at them. They get their names because they look somewhat trees turned upside down, and they show the various constituents branching off. Tree diagrams are used in many disciplines other than linguistics, for example, computer science. They are very good at showing structures that are hierarchical. As language is organized this way, it is a good candidate for representation with tree diagrams. The following is an example of the sort of tree diagram that we will be using for this course:

(28)

Tree diagrams have several advantages over Reed-Kellogg diagrams. They are drawn by following a few simple principles, so you don't have to remember what different line shapes and orientations signify. Despite that simplicity, tree diagrams can represent phrase structure accurately. One practical disadvantage, however, is that they become unwieldy for very long sentences. In comparison, Reed-Kellogg diagrams are more compact. That is, you can more easily fit your analysis of a longer sentence on one page if you use Reed-Kellogg notation than if you use tree notation.

Both tree and Reed-Kellogg diagrams are unreasonably awkward to use in certain contexts, for example in an e-mail or on-line posting where graphics may not be available. In such places, you may also run across other attempts to show syntactic relationships using only ordinary characters. One possibility is to use labeled brackets. The brackets substitute for the lines in showing how the constituents are grouped:

[[Mozart (NP: subj)] [remains (VL) [beloved (AP: s-comp)] [by [contemporary audiences (NP: obj-prep)] (PP: agent) (VP: predicate)] (Clause)].

All the same information is here, but unfortunately, this method tends to lack visual clarity. It's difficult to grasp the constituency of the sentence at a glance the way you can with a diagram. Still another method to indicate constituents uses horizontal lines and labels above (or below). These are a kind of flattened tree diagram, which have the advantage of saving space:[1]


|----------------------- Clause ------------------------------|
             |---------------- VP: predicate -----------------|
                                    |--------PP: agent--------|
|-NP: subj-|         |AP: SComp|        |-----NP: obj-prep----|
    N          VL      Adj            P      Adj          N
  Mozart     remains beloved         by contemporary audiences.

We will generally use tree diagrams in this course. As long as the diagram accurately conveys the sentence structure, however, the exact diagramming scheme we use does not make too much difference.[2] The purpose of diagrams is merely to help us visualize the structure. They are the tools, not the ends, of grammatical analysis.

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Notes

[1] Making such diagrams legible requires that you use a fixed-space font such as Courier rather than the more ordinary proportional fonts used by default in word processors and web pages.

[2] Note that this requirement makes Reed-Kellogg diagrams unsuitable. Their representation of verb phrases in particular is lacking.