The ultimate goal of the present research is the design and fabrication of mass customized chairs using a beech forming process. This paper describes the first stage of the research which is concerned with the development and implementation of a grammar-based design system for Thonet chairs. A corpus of six different chairs designed in the second half of the 19th century was used to infer the grammar, which can account for the design of existing and new chairs in the style. The chairs are composed by four connected beech parts— backrest and rear legs frame, backrest inner area, front legs, and seat, shaped by a forming process. Constraints on the shapes of parts and positions of connections, linked to fabrication and structural performance, respectively, are determinant in the definition of the style. The grammar was converted into a parametric design model to enable its implementation in CATIA. Rapid prototyping of new and existing designs was used to fine tune the parametric model. Results illustrate the potential of a digital design process based on shape grammars, parametric design, and rapid prototyping to enable the mass customization of chairs.