While the vibrating strings of guitars and violins are plainly visible, the sound that they produce in air is invisible. We often associate sound with air because we are used to hearing sounds that reach our ears from the air. We also learn that in the absence of air, sound cannot propagate—movies with sound propagating in outer space, not withstanding. The fact that air is so transparent is not the issue here: Sound travels through liquids such as water and solids such as steel, as well as other gases such as air; nevertheless, we cannot see sound propagating through liquids or solids either. So, what is sound? That is the first subject of this chapter. Once we understand the nature of sound, we will go on to study the modes of vibration of air that is contained in pipes, that is, air columns. These are the basic components for all wind instruments, such as the recorder, flute, and trumpet.