A tripos is divided into two parts: "Part I", which is broadly based, and "Part II", which allows specialization within the student's chosen field. Since a bachelor's degree usually takes three years to complete, either Part I or Part II is two years, and the other one year. The details of this can vary from subject to subject. There is also an optional Part III offered in some subjects, such as the Mathematical Tripos; these are not required to complete a bachelor's degree. Some "Part III" courses allow the student to graduate with both a master's degree and a bachelor's degree: for example, scientific Part III courses allow the student to graduate with an M.Sci. degree in addition to the B.A. degree which all Cambridge graduates receive. The Engineering Tripos is divided into four Parts, each corresponding to one academic year, and leads to the simultaneous awarding of the B.A. and M.Eng. degrees.