In the mid-3rd millennium BC, a new wedge-tipped stylus was introduced which was pushed into the clay, producing wedge-shaped ("cuneiform") signs; the development made writing quicker and easier, especially when writing on soft clay. By adjusting the relative position of the stylus to the tablet, the writer could use a single tool to make a variety of impressions. For numbers, a round-tipped stylus was initially used, until the wedge-tipped stylus was generalized. The direction of writing remained to be from top-to-bottom and right-to-left, until the mid-2nd millennium BC. Cuneiform clay tablets could be fired in kilns to bake them hard, and so provide a permanent record, or they could be left moist and recycled if permanence was not needed. Many of the clay tablets found by archaeologists have been preserved by chance, baked when attacking armies burned the buildings in which they were kept.