A tool for a display system comprises means for superimposing a box on a portion of a buffer of data visible on a display, said box being representative of the size and location of said visible portion with respect to the whole of said buffer. For example, a box of an appropriate size can be superimposed on a portion of a buffer of data displayed in a window at an appropriate location in the window to allow the two-dimensional extent of the visible portion of the buffer with respect to the rest of the buffer of data to be represented in an easily understood manner. It finds particular application as part of a scroll tool. By the provision of means for interacting with the scroll box the user is provided with a user-friendly manner of scrolling around a document in a plurality of directions. This scroll tool is much more convenient to use than conventional scroll bars and does not take up unnecessary screen real estate.