If you plan on sharpening lots of knives using a set of XL stones, you’d better pick up an XL truing stone to keep them flat. If your stone’s dished up like a skateboard it becomes impossible to maintain a consistent angle, and you can't sharpen any more. That’s where the truing stone comes in to play. It is much easier to keep your stones flat than making them flat. We use a truing stone every few knives we sharpen.