A soft sub is a standalone subtitle that is overlaid on top of a video, rather than being written into the video itself. Softsubs are commonly used for fansubs and can be turned on and off during video playback. They exist as independent files wrapped in the video container (MKV, MP4, and MOV), making it very different from hard subtitles whose subtitle text is irreversibly burned in the video frames. Softsubs can be added to videos using a program like WonderFox HD Video Converter Factory Pro, which can help soft code subtitles to MP4, MKV, or MOV without re-encoding, hence keeping the video and audio data intact. Softsubs have the advantage of being adjustable and can be scaled to the video to the viewers liking.