Traditional grammar defines the object in a sentence as the entity that is acted upon by the subject. There is thus a primary distinction between subjects and objects that is understood in terms of the action expressed by the verb, e.g. Tom studies grammar - Tom is the subject and grammar is the object. Traditional theories of sentence structure divide the simple sentence into a subject and a predicate, whereby the object is taken to be part of the predicate. Many modern theories of grammar (e.g. dependency grammars), in contrast, take the object to be a verb argument like the subject, the difference between them being mainly just their prominence; the subject is ranked higher than the object and is thus more prominent.