1. Multiplicity (1)
Nuclei that are close, but are not equivalent, affect each other’s local magnetic fields.
This leads to splitting of signals (spin-spin splitting)
The neighbours’ signal will also be split by the original proton, by the same amount.
Rule of thumb: Signals from nuclei with n non-equivalent neighbours are split into n+1 signals.

2. Multiplicity (2) Proton with one non-equivalent neighbour
The neighbour may be in one of two states: α or β.
This affects Beff for the proton, giving two values for ν
normally half the molecules in a sample will have the neighbour in state α and half in state β
This gives two equal height signals (a doublet).

3. Multiplicity (3) Proton with two non-equivalent neighbours
The neighbours may be in one of four states: αα or αβ, βα or ββ.
As αβ and βα produce the same effect on Beff there are three values for ν
This gives three signals (a triplet) in height ratio 1:2:1.

4. Multiplicity (4) Exceptions to n+1 rule
More than one set of neighbours must be treated sequentially: leading to complex splitting patterns
Protons which are exchanging rapidly in solution e.g. amine or alcohol protons may not couple
All processes which occur faster than about once every half second are “seen” by NMR as averaged. e.g. conformation flipping, rotation of methyl groups J AC AB