Two-dimensional NMR spectroscopy has become an important tool for chemists. Several experiments are possible and new variants are being continually invented, such is the power of the method. The two dimensions are dimensions of time as previously discussed at the beginning of Chapter 8. One of these is already familiar, and is the time domain within which we collect the FID output from the spectrometer and which contains frequency and intensity information. The second dimension refers to the time elapsing between the application of some perturbation to the system and the onset of the collection of data in the first time domain. This second time period is varied in a regular way and a series of FID responses are collected corresponding to each period chosen.