As such, chéngyǔ are fossilized expressions that use the vocabulary and follow the syntactic rules of Literary Chinese. Consequently, they convey information more compactly than normal vernacular speech or writing. They may contain subject and predicate and act as an independent clause (or even twin two-character independent clauses in parallel), or they may play the role of any part of speech in a sentence, acting syntactically as an adjective, adverb, verb, or noun phrase. In both speech and writing, they serve to succinctly convey a complex or multifaceted situation, scene, or concept, and used fittingly and elegantly, they also mark a speaker or writer's erudition.