Deaf people communicate with two hands. Hearing people communicate with only one mouth. What are the implications of this difference for our view of human language? The proposed research aims to approach this central issue by addressing a number of new and concrete questions, using highly innovative methods. Linguistic models are generally based on the fact that spoken language structures are linear and hierarchically structured. The symmetry of the two hands in signed languages makes room for abundant simultaneity, even though our motor system imposes severe constraints on the independent action of the two hands. We know that deaf people rarely utter two different sentences at the same time (one with the left and one with the right hand), but there are indications that from below the level of single signs up to the structuring of discourse, the two hands contribute to communication in a fashion unparalleled in spoken languages. The two hands can be mere copies of each other, but in many well-described cases the second hand is not redundant and both hands can perform independent linguistic functions. This proposal focuses on three questions that target different components in Nederlandse Gebarentaal (NGT, Sign Language of the Netherlands): discourse, syntax, and phonology. 1. What kind of information does a second hand contribute to signed communication in longer utterances (discourse)? 2. Which types of utterances in which a second hand occurs (subordination, coordination, parenthesis, apposition) can be distinguished in signed sentences? 3. What are the constraints on the one-handed articulation of two-handed signs, which is often necessary to create simultaneous constructions? To answer these questions, the most recent technical possibilities will be employed. Our corpus of NGT (Sign Language of the Netherlands), one of the first and largest digital sign language corpora in the world, will serve as the central resource for addressing all questions.