Illustrated Glossary of Organic Chemistry

First-order coupling: In NMR spectroscopy, spin-spin coupling in which the energy differences between the spin states are equal. Results in first-order splitting.

First order coupling J_ab = J_ac		Non-first-order coupling J_ab ≠ J_ac

First-order splitting results. The n+1 rule is obeyed. A triplet is observed.		Non-first-order splitting results. The n+1 rule is not obeyed. A doublet of doublets is observed.