Ply Gem Stone True Stack modular stone provides a dry stack style with time-saving installation. The rectangular pieces come in three different lengths –9, 13 and 18 in. long –that fit tightly together, with unique face textures. On the back of the stone, a 1/4-in. groove creates a clean 1/2-in. gap when the stones are laid together, making it easy to fill and blend mortar, and forming seamless lines between the pieces.