Writers often make the assumption that their readers share intellectual background --a core of symbols, characters, metaphors and meanings. This may be true for a writer's peers and contemporaries, but with social distance and the passage of time a text may become a puzzle of obscure references and semantic ambiguities. A reader may require guidance to decode an author's messages and thread the web of allusions.
Several distinct realms of allusion can be identified:

• Classical allusions --to characters and episodes in Greek and Roman literature, mythological and historical
• Biblical allusions
• Symbols
• References to current religious and intellectual controversies
• Phrase and Fable compendia