The creation of prototypes and their iterative adaptation are important stages in the time-consuming development process of a multimodal system. This reality has brought about the appearance of many specialized toolkits intended for facilitating the prototyping of multimodal systems. Using these toolkits, some functionalities of the intended prototype can be specified by means of a visual language instead of programming code. This article reports on a comparative study of a representative set of graphical toolkits for rapid prototyping of multimodal systems. It presents a set of criteria to identify the scope of a toolkit. According to their scopes, toolkits can be clustered into three classes called flow-based, state-based and token-based. Toolkits within each class do not only offer similar benefits to their users, but also exhibit resemblance in their underlying visual languages.