spin-spin splitting
Look closely at a real NMR signal and you often see that it is not one clean peak but a little cluster — a doublet of two peaks, a triplet of three, a quartet of four. This splitting is not noise; it is the molecule telling you about a hydrogen's neighbors. Spin-spin splitting is the most information-rich of the four NMR clues, because it reveals not just what a hydrogen is, but who sits next to it.
The mechanism is a kind of magnetic eavesdropping. Each neighboring hydrogen is itself a tiny magnet that can point either with or against the main field; the hydrogen you are watching feels its neighbors' little fields added to or subtracted from the big one, so its signal splits into several slightly different lines. The simple bookkeeping is the n+1 rule: a hydrogen with n equivalent neighboring hydrogens is split into n+1 peaks. So a hydrogen next to a CH3 (three neighbors) becomes a quartet (3+1), and a hydrogen next to a CH2 (two neighbors) becomes a triplet (2+1). The pattern points like a finger at the adjacent group.
This is why splitting is so powerful in piecing structures together. A classic giveaway is an ethyl group, CH3-CH2-: the CH3 appears as a triplet (split by the two CH2 hydrogens) and the CH2 as a quartet (split by the three CH3 hydrogens), a paired triplet-quartet signature you learn to recognize at a glance. By reading each signal's multiplicity, a chemist maps out which carbons are bonded to which, stitching the fragments into a complete skeleton.
In 1,1,2-trichloroethane (CHCl2-CH2Cl), the single CH hydrogen, having two neighbors on the CH2, appears as a triplet, while the two CH2 hydrogens, having one neighbor, appear as a doublet — a clean triplet/doublet pair predicted by the n+1 rule.
n neighboring hydrogens split a signal into n+1 peaks, revealing the adjacent group.
The n+1 rule counts neighbors on adjacent atoms, not the hydrogens producing the signal itself, and it assumes those neighbors are all equivalent. Equivalent hydrogens on the same carbon do not split each other, and non-equivalent neighbors give more complex patterns the simple rule does not cover.