Many common operations can be performed with a single keystroke. In this case it's a shortcut for an operation
that can also be performed otherwise, for example with a button or a menu item.
There are also a few infrequent operations that can only be performed with a key.
The effect of some of the keys, like '+', is empowered by the Shift key (⇧) and decreased by the Control key (ctrl).
For example: the plus key + increases the vertical scale by a factor of 2,
the combination Shift-plus by a factor of 5 and Control-plus by a factor of 1.1.
Holding a key down, its action is continually repeated.