Steering law is an excellent performance model for trajectory-based tasks in GUIs. However, since the original law was proposed, it has been examined only in a graphical environment of spatially decoupled style. Moreover, pointer size has been limited to a small one, and the trajectory width of the trajectory has also been limited to a certain size. To solve this problem, in this paper we discuss the extension of the original steering law in order to apply the law to a wider range of environments. We prove the steering law in an environment of spatially coupled style. We explore three conditions of the pointer and trajectory: a sufficiently small pointer and a trajectory of certain width; a pointer of certain size and a narrow trajectory, and, a pointer of certain size and a trajectory of certain width. The experimental results show that the steering law is valid in an environment of spatially coupled style.