Multiplicity refers to the number of signals in the NMR spectrum that correspond to a single proton. A proton can be \"split\" by neighboring protons. This means that the nuclear spin of one proton causes the spin of another to resonate at different frequencies. In the simplest case, a proton with one neighbor shows up as a doublet (two peaks) instead of one single line. The multiplicity of a signal is simply the number of such lines that correspond to a single proton (or a single set of equivalent protons). Solution Multiplicity refers to the number of signals in the NMR spectrum that correspond to a single proton. A proton can be \"split\" by neighboring protons. This means that the nuclear spin of one proton causes the spin of another to resonate at different frequencies. In the simplest case, a proton with one neighbor shows up as a doublet (two peaks) instead of one single line. The multiplicity of a signal is simply the number of such lines that correspond to a single proton (or a single set of equivalent protons)..