This chapter describes how the 21-cm line is used to study the high-z Universe. It introduces the spin-flip or the hyperfine line—a transition driven by the interaction of the spins of the proton and electron, whose relative directions affect the energy of the electron's orbit. An atom in the upper state eventually undergoes a spin-flip transition, emitting a photon with a wavelength of 21 cm. As the chapter shows, this transition is extremely weak, so the effective intergalactic medium (IGM) optical depth is only of the order of 1 percent: this makes the entire neutral IGM accessible during the cosmic dawn. Moreover, the transition energy is so low that it provides a sensitive thermometer of the low-temperature IGM, and as a low-frequency radio transition, it can be seen across the entirety of the IGM against the cosmic microwave background.
