The fundamental aim of computer-aided learning (CAL) is to communicate with the student. The most important design feature is the user interface. In our system, this involves the use of a keyboard and mouse for inputting mathematical expressions and controlling the behaviour of the software. This article describes a software tool designed as an input tool (EqEditor) for an intelligent tutoring system (ITS). With respect to the benefits of graphical user interface (GUI) design, the principles of good design have been considered as a guideline for designing the interface. The system was designed for the World Wide Web using an applet written in Java, and could be used as a distance learning tool for learning and teaching mathematics.