This book is about the sounds we use when we speak (as opposed to the sounds we make when we’re doing other things). It’s also about the various kinds of relationship that exist between the sounds we use. That is, it’s about ‘phonetics’ – the physical description of the actual sounds used in human languages – and it’s about ‘phonology’ – the way the sounds we use are organised into patterns and systems. As speakers of a particular language (English, say, or Hindi or Gaelic or Mohawk) we obviously ‘know’ about the phonetics and phonology of our language, since we use our language all the time, and unless we are tired or not concentrating (or drunk), we do so without making errors. Furthermore, we always recognise when someone else (for example a non-native speaker) pronounces something incorrectly. But, equally obviously, this knowledge is not something we are conscious of; we can’t usually express the knowledge we have of our language. One of the aims of this book is to examine some ways in which we can begin to express what native speakers know about the sound system of their language.