In addition to the syntactic tests just described experimental evidence has shownthet speakers do not represent sentences as strings of words but rather in terms of constituents. In these experiments subjects listento to sentences that have clicking noises inserted into them at random points. In some cases the click occurs at a constituent. boundary and in other sentences the click is inserted in the middle of a constituent. The subjects are then asked to report where the click occurred. There were two important results: (1) subjects noticed the click and recalled its location best when it occurred at a major constituent boundary (e.g. between the subject and predicate)and (2) clicks that occurred inside the constituent were reported to have occurred between constituents. In other words subjectto displaced the clicks and put them at constituent boundaries. These results show that speakers perceive sentences in chunks corresponding to grammatical constituents.
Every sentence in a language is associated with one or more constituent structures. If a sentence has more than one constituent structure it ambiguous and each tree will correspond to one of the possible meaninges. For example the sentence bought an antique desk suitable desk suittable for a lady with thick legs and large drawers has two phrase structure trees associated with it. In one structure the phrase [a lady with thick legs and large drawers] forma a constituent. For example it could stand alone in answer to a question who did you buy an antique desk for? In its second meaning the phrase a desk for a lady and thus the structure is [[a desk fos a lady][with thick thick legs and large drawers]].