Proton-proton couplings arise from influences on the proton’s magnetic field from the magnetic field of neighbouring protons. Being capable to recognize among the vast array of coupling patterns can facilitate the time spent on interpreting a 1H NMR.
The diagram below illustrates two coupling patterns: a triplet of doublets (td) and a doublet of triplets (dt). The J coupling constants for both multiplets are measured at 3.2 and 8.3 Hz. The difference is the left multiplet (td) exhibits two couplings of 8.3 Hz and one of 3.2 Hz and the right multiplet (dt) exhibits one coupling of 8.3 Hz and two of 3.2 Hz.