This is not a question of what the function of each button is. It is perfectly simple to understand that the AC button clears everything and the C button clears your last entry. What I am concerned about is the practical uses of each button. Is there any difference?
Probably the most worn out button on any calculator. Photo courtesy of http://www.flickr.com/photos/jaybee3/
I’m pretty sure that most calculator users will recognise the familiar compulsion to jab at the AC button a minimum of four or five times before proceeding with a new calculation. It is simply impossible to press it only once. What if it hasn’t really cleared everything? Then every subsequent calculation will be incorrect and the very instrument that you use to ensure that you will never again be required to use long multiplication will have tricked you into inaccuracy. Thus it is necessary to prod the AC button over and over until you are really certain that everything is all clear. Then maybe once more to be sure.
The C button on the other hand is plainly redundant. Nobody uses it for the very reason outlined above. The problem lies in the uncertainty of how much of the calculation has cleared. Calculators obviously were not paying attention in school maths lessons, as they refuse to show their working out. It is impossible to determine exactly what you have cleared and thus where you will resume the calculation. Its use will invariably lead to a thorough revisiting of the AC button so nobody ever uses it. It takes up valuable room on the keypad that could be used for a button that instantly recalls the 5318008 needed to spell out boobies. I imagine that obsessive compulsives have a very difficult time of things in maths exams.
The Badgerwolf 