This document discusses the linguistic concepts of dialect, register, and style. It defines register as varieties of language defined by their social use, such as the registers of scientific or religious language. Dialect refers to varieties according to the user. The document explores the relationships and overlaps between these concepts. It examines factors that influence register, such as formality, topic, and social roles. Models of analyzing registers along dimensions like field, mode, and tenor are discussed. The principles of stylistic variation and how style relates to formality are also summarized.