Non-destructive editing (NDE) refers to editing techniques or features that allow the original contents to be preserved, while also allowing for visible changes to be made. In practice, painting and retouching tools, filters, and adjustments make what are called destructive edits—certain pixels are altered. Further edits will call on the newly-changed data, as the original data is no longer available. Though many image editing programs have an Undo command, use of the command is often limited to one edit, with the next use either being not available or "undoing the undo". Unless a backup copy was made in advance, saving the image will overwrite the original image data forever.