Multimedia synchronization comprises both the definition and the establishment of temporal relationships among media types. The presentation of 'in sync' data streams is essential to achieve a natural impression, data that is 'out of sync' is perceived as being somewhat artificial, strange, or even annoying. Therefore, the goal of any multimedia system is to enable an application to present data without no or little synchronization errors. The achievement of this goal requires a detailed knowledge of the synchronization requirements at the user interface. The paper presents the results of a series of experiments about human media perception that may be used as 'quality of service' guidelines. The results show that skews between related data streams may still give the effect that the data is 'in sync' and gives some constraints under which jitter may be tolerated. The author uses the findings to develop a scheme for the processing of nontrivial synchronization skew between more than two data streams.