This chapter discusses how different languages may have different inventories of sounds, and some of the ways that sounds are produced and perceived. It describes in more depth the ways that linguists study sound production and perception. Phoneticians are interested in how sounds are produced and perceived. Phoneticians describe sounds in terms of where they are produced in the vocal tract, and in how they are produced and how the air is stopped or constrained. Articulatory phonetics is the study of how we produce sounds. Sounds are also described by the manner of articulation. Sometimes phonological rules involve insertion of a sound, as when a word like realtor has a vowel added between the /l/ and the /t/, resulting in a pronunciation like real-a-tor. Auditory phonetics is the study of how sounds are perceived by the human ear or recognized and processed by the human brain.