The person using the computer keeps a similar structure inside his head - the mental model of the computer. The two structures are not exactly the same, of course, for then the machine would not be needed at all. This is important as a person can only make decisions based on the model, for the internal state is not directly accessible. Most application programs, however, include both functions at the same time. These solutions were practical when the number of users was reduced, but the virtual explosion of personal computing in the last decade has made these costs unacceptable and has increased the interest in UI developments.
Modes have several problems.
Operating modes greatly increase the complexity of a system, making the user type many mode-changing commands before he can execute the command he is really interested in. The problem is that there is no way to switch to that mode and then come back to the first one. The use of modes imposes a rigid structure in the man-machines dialog, which makes the programmer's life easier, but not the user's. Windows and Menus Figure 3.1. The keyboard was freed from the many operating modes, being reduced to just text entry.