Marine signal flags are a complete set of flags use to communicate on the water. You can use them to spell out any message. There are some flags that have other meanings, especially if they are used with other flags. For example, "DX" means that this ship is sinking. (I guess it won't be flying this flag for too long), a "C" means "yes", and "N" means "no". We have put together a slide show with some of the letters and the meaning behind them. I think it would be difficult to communicate well with flags.