Cato ‒ a guided user interface for different CAS. CATO is a new user interface, written in Java, and developed by the author as a response to the significant difficulties faced by those engineers and students who only sporadically use computer algebra systems (CAS). The usage of CAS in mathematical lectures should be an integral part of mathematical instruction. However, difficulties arise for those students who have classes that meet only once or twice a week, and therefore use CAS only irregularly. Such difficulties are compounded when two different CAS must be used in instruction. The author has developed a guided user interface (GUI) which translates commands into the languages of different CAS. His intention in so doing has been to develop intuitive operability for CAS. For example, it is common in existing CAS that commands with more than one parameter have their own input windows with commentated input rows for every parameter. In response, the newly developed surface CATO orders the parameters, and uses the right brackets and separators. Below, the author demonstrates the usage of CATO with maxima and with the symbolic Math. Toolbox of MATLAB.