Research into avian vocal duets has revealed much about the social interactions of mated pairs, including how communication signals mediate important components of fitness such as reproductive success and resource-holding ability. Excellent reviews have been written on avian duets, but none have quantified the prevalence of different duet features and functions, or tested for potential links between structure and function. Here we have compiled a thorough list of studies at the species level and investigated whether there are trends in duet form, trends in duet function and matches between form and function. We have also provided recommendations for reporting of duet structure to help standardize future data collection and facilitate comparative analyses. We found that duet vocalizations are typically tightly coordinated (69% of 33 species), initiated by both partners (80% of 35 species), and have sex-specific contributions (73% of 48 species). Duets vary substantially in structure both across and within species, and this flexibility in duet form may contribute to their use in multiple contexts. Among the well-studied species (functions well supported by experimental evidence and studied post-1983), duets are usually multifunctional, with functions including joint resource defence, signalling commitment, maintaining contact, ensuring reproductive synchrony and mate-guarding. Levels of support for different hypotheses have changed over time as theory within the field developed, but overall, the best-supported function of duets across avian species (81% of 26 species) has been joint resource defence. In all but one of these 26 species, duets are sometimes cooperative (meaning that duet participation benefits both duetting partners), and they often produce multiple benefits. We found associations between form and function, in that duets directed to non-pair individuals have higher amplitude and are more likely to have sex-specific notes than duets directed within the pair. We see the field moving away from labelling duets as representing ‘cooperation’ or ‘conflict’ and moving towards a general multifunctionality framework within which researchers test multiple hypotheses for duet function under the assumption that both duet form and function can vary moment to moment and that such flexibility can provide multiple benefits.