Nuclear magnetic resonance (NMR) spectroscopy can measure radio-frequency Zeeman transitions of proton spins in a magnetic field. It is more convenient to sweep the magnetic field through the resonances at a fixed frequency, typically 60 MHz. The resonances are sensitive to the chemical environment of nonequivalent protons, an effect known as the chemical shift. A classic example is the ethanol molecule CCOH, which shows three chemically-distinct hydrogen atom sites, thus three NMR peaks with intensity ratios 3:2:1. The relevant parameter is , representing the fractional deviation of the chemical shift measured in parts per million (ppm) from that of tetramethylsilane (TMS), a convenient standard assigned the reference value . A small amount of TMS is often added to the sample being measured to calibrate the -scale.