The interface is designed primarily for pen input, however a mouse can still be used as all operations use at most one button. This single button operation is achieved with the pen by pressing it against the tablet.
While some pens do offer additional buttons on their barrel or at the top end of the pen, it can be inconvenient or difficult to use these with precision. Thus, all functions in addition to basic drawing of strokes, such as editing operations, should either be conveyed to the system through the use of menus, toolbars, keystrokes, or ideally specialised gestures.
There are a number of ``standard'' editing gestures used in pen based computing. Books such as Microsoft's ``The Windows Interface Guidelines for Software Design'' describe a set of suggested gestures to use. Some of these gestures are based on ``traditional'' proofreading marks, or reasonable abbreviations. For example, as shown in Figure , a circled ``u'' means undo.
With a system based on a character recogniser, it is inevitable that recognition errors will occur as the user writes their formulae, no matter how advanced the recogniser is. Humans often have trouble reading each other's writing, and, at times, people are even unable to read their own. Thus, a simple method for correcting errors needs to be provided.