The E.COSY experiment is one of the best ways to determine proton-proton coupling constants. By combining double quantum with triple or higher quantum spectra, the resulting cross peak multiplet pattern is simplified, relative to DQF-COSY spectra. The reduced number of mulitplets results in less cancellation of antiphase components and allows easier interpretation. Below is an E.COSY spectrum of dibromopropionic acid where J21=12 Hz, J31=5 Hz, and J23=13 Hz. Consider the H3-H1 cross peak (light blue peaks are positive and yellow are negative) The active coupling, J31 is responsible for this peak and gives an antiphase doublet in both dimensions separated by 5 Hz, creating a square, as shown. Passive couplings create another square in phase with the first and offset by the coupling. In this case, the passive coupling from H1 to H2, creates another square offset in F2 by J21, and offset in F1 by the passive coupling to H3, J23, as shown. A disadvantage of E.COSY is the reduced signal-to-noise of higher quantum spectra.