predicate, e.g. "I have a ball." In this sentence, one can change the persons, e.g. "We have a ball." However, a minor sentence is an irregular type of sentence that does not contain a main clause, e.g. "Mary!", "Precisely so.", "Next Tuesday evening after it gets dark." Other examples of minor sentences are headings (e.g. the heading of this entry), stereotyped expressions ("Hello!"), emotional expressions ("Wow!"), proverbs, etc. These can also include nominal sentences like "The more, the merrier." These mostly omit a main verb for the sake of conciseness but may also do so in order to intensify the meaning around the nouns. Sentences that comprise a single word are called word sentences, and the words themselves sentence words. Length The 1980s saw a renewed surge in interest in sentence length, primarily in relation to "other syntactic phenomena". One definition of the average sentence length of a prose passage is the ratio of the number of words to the number of sentences. The textbook Mathematical linguistics, by András Kornai, suggests that in "journalistic prose the median sentence length is above 15 words". The average length of a sentence generally serves as a measure of sentence difficulty or complexity. In general, as the average sentence length increases, the complexity of the sentences also increases. Another definition of "sentence length" is the number of clauses in the sentence, whereas the "clause length" is the number of phones in the clause. Research by Erik Schils and Pieter de Haan by sampling five texts showed that two adjacent sentences are more likely to have similar lengths than two non-adjacent sentences, and almost certainly have a similar length when in a work of fiction. This countered the theory that "authors may aim at an alternation of long and short sentences". Sentence length, as well as word difficulty, are both factors in the readability of a sentence; however, other factors, such as the presence of conjunctions, have been said to "facilitate comprehension considerably". See also Clause Constituent Discourse Periodic sentence Sentence arrangement Sentence function Syntax T-unit Utterance References Semantic units Statements Syntactic entities