An automatic sentence diagrammer is an application or program that uses Reed-Kellogg diagrams to illustrate the structure of a sentence. Sentence diagrams are used for English learners to understand how grammar works.
A Reed-Kellogg diagram, introduced in 1877, separates the subject and predicate of a sentence with a vertical line. In more complex sentences, the diagram uses branches off the main sentence to explain which words are modified. For example, the diagram of the sentence "Darwin works quickly" shows "Darwin," the subject, on the left side of the diagram. The primary part of the predicate, the verb "works," appears on the right side of the diagram. The adverb "quickly" modifies the verb, so the diagram shows "quickly" on a branch coming out from under the verb.