After doing a small amount of research on keyboards, I discovered key matrices. I also observed the usage of diodes to prevent some problems with keystroke recognition amongst people at this forum. However, I still don't understand how certain combinations of key-presses can be made to work by simply adding diodes.
This website was the easiest to comprehend, but I suspect there are some mistakes made in the graphics:
http://pcbheaven.com/wikipages/How_Key_Matrices_Works/The image in concern is this. I have edited it a little to indicate the button I am concerned with. Three buttons, B2, B3 and C2, are already pressed. The button that will be pressed is the bottom-right one, C3. However, current would already be flowing through pins C and 3. How does the controller distinguish whether C3 has been pressed or not, and how do diodes make this possible? I know what they are, they allow current in one direction only.
Also, a bigger example, how would a controller distinguish whether the central (B2) key has been pressed if every other key (all the ones surrounding it) are already pressed?
I'm getting the feeling that I'm missing something very obvious...
EDIT: Added picture.