The materials used in referential communication tasks are typically visual and nonverbal. That is, diagrams, drawings, photographs, and pictures are generally selected or designed for use in the tasks, whereas textual material, or any type of written language, is mostly avoided. Physical objects or parts of objects (to be identified or assembled) are also used, as well as toys and components of construction games. Although it is rarely discussed, the primary assumption underlying these choices seems to be tied to a preference for eliciting the participants' own language, in a fairly spontaneous way, rather than prompting or priming them to use provided language. It also seems to be considered an advantage that such materials art proficiency neutral, to the extent that no participant is required to cope with potentially unfamiliar or puzzling written language in the materials. In this way, problems related to literacy levels are also minimized. With regard to spoken language proficiency, these materials allow participants to use whatever L2 knowledge they possess (or can recall) during task performance.