Consider air trapped inside a tube that is open at one end and closed at the other. If a wave impinges on it, it reflects, producing standing waves. The boundary conditions are antinodes at the open end, where air molecules are free to vibrate, and nodes at the closed end, where they are not. If both ends were closed, the first possible mode would have nodes at both ends. But here, the first possible mode can be obtained by stretching this pattern. The tube's length equals one-fourth the wavelength of the fundamental mode. For the first overtone, the length is equal to three-fourths the wavelength. For the second overtone, it is five-fourths the wavelength, and so on. So, the wavelengths of the standing wave modes are found to follow a mathematical pattern given by n, an odd integer. The corresponding frequencies, or the harmonics, are obtained in terms of wave speed.