Bonnell springs are the classic type of springs, and probably the best known. The first interior-sprung mattresses appeared in the 1890's and were named after their inventor, Bonnell. Their single springs had a "waisted" shape (or a shape like an hour-glass) which were joined by wire coils. The resulting springs gave a high degree of stability and durability. The waisted shape also gave them progressive springiness so that increased loading resulted in increased resistance. In general Bonnell mattresses are somewhat firm to lie on.