Dry placed stone is, no doubt, the earliest form of masonry; it consists of the simple stacking of natural stones, one upon another, without mortar. As stone cutting tools began to be employed for working stone, early dry stone masons developed techniques of laying stones to make structures more stable and weather resistant. Egypt's great pyramids are the largest and most recognized dry stone structures, but there are numerous examples of ancient dry stone masonry all around the world that are still stable and useful after hundreds or even thousands of years.