In pedagogy and theoretical syntax, a sentence diagram or parse tree is a pictorial representation of the grammatical structure of a sentence. The term "sentence diagram" is used more in pedagogy, where sentences are diagrammed. The term "parse tree" is used in linguistics (especially computational linguistics), where sentences are  parsed. The purpose of sentence diagrams and parse trees is to have a model of the structure of sentences. The model is informative about the relations between words and the nature of syntactic structure and is thus used as a tool to help predict which sentences are and are not possible.