Genre () is any form or type of communication in any mode (written, spoken, digital, artistic, etc.) with socially-agreed-upon conventions developed over time. In popular usage, it normally describes a category of literature, music, or other forms of art or entertainment, whether written or spoken, audio or visual, based on some set of stylistic criteria, yet genres can be aesthetic, rhetorical, communicative, or functional. Genres form by conventions that change over time as cultures invent new genres and discontinue the use of old ones. Often, works fit into multiple genres by way of borrowing and recombining these conventions. Stand-alone texts, works, or pieces of communication may have individual styles, but genres are amalgams of these texts based on agreed-upon or socially inferred conventions. Some genres may have rigid, strictly adhered-to guidelines, while others may show great flexibility.