This is a book about phonology or sound patterns of the world's languages. In this book I present a new synthesis of historical and non-historical accounts of sound patterns. Anyone who studies phonology soon comes to realize that many sound patterns occur with more-than-chance frequency. Other sound patterns are quite rare. In this book, I provide evidence that recurrent sound patterns are the transparent result of recurrent sound change, where sound change is a direct consequence of the indirect transmission of language across generations. Other recurrent sound patterns reflect direct inheritance from a mother tongue. Rare sound patterns are either the result of uncommon sound changes, or result from the combined effects of independent sound changes. Certain sound changes are more common than others because they are rooted in the way we hear and speak. By attributing common sound patterns to phonetically based sound change, there is no need to encode the same phonetic explanations in synchronic grammars. Synchronic grammars model speakers' knowledge of language. In the realm of sound patterns, synchronic grammars include seemingly universal categories of features, segments, syllables, and other prosodic categories. At the same time, they encode many language-specific sound patterns. By viewing recurrent sound patterns as instances of parallel or convergent evolution in the domain of speech perception and production, synchronic grammars can be modeled as pure expressions of grammatical knowledge.