When deaf signers of Sign Language of the Netherlands (NGT, Nederlandse Gebarentaal) sign with each other, they often combine their signing with mouthings: typically silent articulations of lexical items from spoken Dutch. Using a large corpus of (semi-)spontaneous signing by deaf native signers, this dissertation investigates the co-occurrence of these mouthings with NGT signs, how they vary between tokens of the same sign, and how they combine with signs and sentences. The relation between mouthings and signs appears to be not very tight. Although most signs are accompanied by so-called standard mouthings, there is variation in the way mouthings are temporally reduced, variation in the use of either a mouthing or a mouth gesture with a sign, and to a minor extent variation in the choice of Dutch lexical items that are used as mouthings. Mouthings do not necessarily have a one-to-one relation with
the signs they co-occur with. They can spread over adjacent signs to combine and group multiple signs under one mouthing, or they can occur in the sign stream without an accompanying sign, fulfilling a role that could have been taken by a manual sign as well. The general conclusion is that mouthings are a paramount feature in NGT conversation, and that they can be analysed as instances of code-blending, the simultaneous mixing of languages in different modalities. Understanding the role of the mouth offers substantial new insights in the cross-modal interaction between two languages.