C2 proton of trans-cinnamaldehyde
Multiple coupling of C2 proton gives “doublet of doublets”
Splitting pattern illustrated by tree diagram
C3 proton splits C2 proton signal into doublet (J2-3 = 12 Hz) which is split by C1 aldehyde proton into new doublets (J1-2 = 6 Hz) producing four-line spectrum