Evidence suggests that sketches of seen objects exhibit a structure consistent with the volumetric parts of the recalled objects. Occasionally, sketching structure does not match volumetric part structure. Top-down cognitive factors, perception, or a combination of both could trigger switching of drawing behaviour. From the evidence, is concluded that uncertainty is the primary factor triggering change in drawing structure. Nevertheless, the data indicate that sketches can engender uncertainty by revealing errors in recollection, through recognition failures or by promoting reasoning about depicted objects, and can facilitate the resolution of uncertainty by stimulating recall.