Mouthings and mouth gestures are omnipresent in Sign Language of the Netherlands (NGT). Mouthings in NGT are mouth actions that have their origin in spoken Dutch, and are usually time aligned with the signs they co-occur with. Frequently, however, they spread over one or more adjacent signs, so that one mouthing co-occurs with multiple manual signs. We conducted a corpus study to explore how frequently this occurs in NGT and whether there is any sociolinguistic variation in the use of spreading. Further, we looked at the circumstances under which spreading occurs. Answers to these questions may give us insight into the prosodic structure of sign languages. We investigated a sample of the Corpus NGT containing 5929 mouthings by 46 participants. We found that spreading over an adjacent sign is independent of social factors. Further, mouthings that spread are longer than non-spreading mouthings, whether measured in syllables or in milliseconds. By using a relatively large amount of natural data, we succeeded in gaining more insight into the way mouth actions are utilised in sign languages.