Any operation that modifies the document can be undone via Edit/Undo or Ctrl+Z, and redone via Edit/Redo or Ctrl+Y. Undo is unlimited by default, but it can also be limited to a specific number of levels. Each document has its own list of previous states, or undo history, and these histories can become large, particularly when big files are edited. Consequently undo states are stored on disk by default, though they can alternatively be stored in memory to improve performance; see disk threshold. The undo history can be viewed and traversed via the history bar.