Shimming is the process of adjusting the shim currents in order to improve the homogeneity of the bulk magnetic field. Magnetic inhomogeneity is spacial variations in the field arising from imperfections in the magnetic coil or from induced deviations in the magnetic field due to the presence of a sample within the magnet. Shimming is done on every sample inserted into the magnet in order to achieve the narrowest linewidth possible. In modern spectrometers shimming is done by varying the amount of current passing through different shim coils each with a specific geometry.