A type of NMR that uses pulses and Fourier transform methods such as COSY to disentangle the overlapping fine structure of different NMR lines by using two axes to represent the data. This technique is particularly useful for complex molecules such as proteins and nucleic acids as it gives a map of chemical shifts and spin couplings, leading to an understanding of the chemical bonds in the molecules. Two-dimensional NMR was initiated in the 1970s and has greatly expanded the scope of NMR.